Spin dependent tunneling memory

ABSTRACT

A digital data memory having a bit structure in a memory cell based on a dielectric intermediate separating material with two major surfaces having thereon an anisotropic ferromagnetic thin-film of differing thicknesses. These bit structures are fabricated within structural extent limits to operate satisfactorily, and are fabricated as series connected members of storage line structures. A corresponding conductive word line structure adjacent corresponding ones of these memory cells is used for selecting or operating them, or both, in data storage and retrieval operations.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is continuation of Application Ser. No. 09/435,598,filed Nov. 8, 1999 now U.S. Pat. No. 6,275,411 entitled “Spin DependentTunneling Memory”, which is a continuation-in-part of a pplication Ser.No. 08/679,183, filed Jul. 12, 1996 now U.S. Pat. No. 6,168,860 for“Magnetic Structure with Stratified Layers” which is a continuation ofapplication Ser. No. 08/096,765, filed Jul. 23, 1993 now abandoned; andis also a division of application Ser. No. 08/965,333, filed Nov. 6,1997, now U.S. Pat. No. 6,021,065, granted Feb. 1, 2000, that claimspriority from Provisional Patent Application No. 60/030,236, filed Nov.8, 1996 for “Spin Dependent Tunneling Memory”, and is acontinuation-in-part of application Ser. No. 08/704,315, filed Sep. 6,1996 for “Giant Magnetoresistive Effect Memory Cell”, now U.S. Pat. No.5,949,707, granted Sep. 7, 1999.

BACKGROUND OF THE INVENTION

The present invention relates to ferromagnetic thin-film structuresexhibiting relatively large magnetoresistive characteristics and, moreparticularly, to such structures used for the storage and retrieval ofdigital data.

Many kinds of electronic systems make use of magnetic devices includingboth digital systems, such as memories, and analog systems such asmagnetic field sensors. Digital data memories are used extensively indigital systems of many kinds including computers and computer systemscomponents, and digital signal processing systems. Such memories can beadvantageously based on the storage of digital symbols as alternativestates of magnetization in magnetic materials provided in each memorystorage cell, the result being memories which use less electrical powerand do not lose information upon removals of such electrical power.

Such memory cells, and magnetic field sensors also, can often beadvantageously fabricated using ferromagnetic thin-film materials, andare often based on magnetoresistive sensing of magnetic states, ormagnetic conditions, therein. Such devices may be provided on a surfaceof a monolithic integrated circuit to provide convenient electricalinterconnections between the device and the operating circuitrytherefor.

Ferromagnetic thin-film memory cells, for instance, can be made verysmall and packed very closely together to achieve a significant densityof information storage, particularly when so provided on the surface ofa monolithic integrated circuit. In this situation, the magneticenvironment can become quite complex with fields in any one memory cellaffecting the film portions in neighboring memory cells. Also, smallferromagnetic film portions in a memory cell can lead to substantialdemagnetization fields which can cause instabilities in themagnetization state desired in such a cell.

These magnetic effects between neighbors in an array of closely packedferromagnetic thin-film memory cells can be ameliorated to aconsiderable extent by providing a memory cell based on an intermediateseparating material having two major surfaces on each of which ananisotropic ferromagnetic memory thin-film is provided. Such anarrangement provides significant “flux closure,” i.e. a more closelyconfined magnetic flux path, to thereby confine the magnetic fieldarising in the cell to affecting primarily just that cell. This resultis considerably enhanced by choosing the separating material in theferromagnetic thin-film memory cells to each be sufficiently thin.Similar “sandwich” structures are also used in magnetic sensors.

In the recent past, reducing the thicknesses of the ferromagneticthin-films and the intermediate layers in extended “sandwich”structures, and adding possibly alternating ones of such films andlayers, i.e. superlattices, have been shown to lead to a “giantmagnetoresistive effect” being present in some circumstances. Thiseffect yields a magnetoresistive response which can be in the range ofup to an order of magnitude or more greater than that due to the wellknown anisotropic magnetoresistive response.

In the ordinary anisotropic magnetoresistive response, varying thedifference occurring between the direction of the magnetization vectorin a ferromagnetic thin-film and the direction of sensing currentspassed through that film leads to varying effective electricalresistance in the film in the direction of the current. The maximumelectrical resistance occurs when the magnetization vector in the fieldand the current direction therein are parallel to one another, while theminimum resistance occurs when they are perpendicular to one another.The total electrical resistance in such a magnetoresistive ferromagneticfilm can be shown to be given by a constant value, representing theminimum resistance, plus an additional value depending on the anglebetween the current direction in the film and the magnetization vectortherein. This additional resistance has a magnitude characteristic thatfollows the square of the cosine of that angle.

Operating magnetic fields imposed externally can be used to vary theangle of the magnetization vector in such a film portion with respect tothe easy axis of that film. Such an axis comes about in the film becauseof an anisotropy therein typically resulting from depositing the filmduring fabrication in the presence of an external magnetic fieldoriented in the plane of the film along the direction desired for theeasy axis in the resulting film. During subsequent operation of thedevice having this resulting film, such operational magnetic fieldsimposed externally can be used to vary the angle to such an extent as tocause switching of the film magnetization vector between two stablestates which occur for the magnetization being oriented in oppositedirections along the film's easy axis. The state of the magnetizationvector in such a film can be measured, or sensed, by the change inresistance encountered by current directed through this film portion.This arrangement has provided the basis for a ferromagnetic,magnetoresistive anisotropic thin-film to serve as a memory cell.

In contrast to this arrangement, the resistance in the plane of aferromagnetic thin-film is isotropic for the giant magnetoresistiveeffect rather than depending on the direction of the sensing currenttherethrough as for the anisotropic magnetoresistive effect. The giantmagnetoresistive effect involves a change in the electrical resistanceof the structure thought to come about from the passage of conductionelectrons between the ferromagnetic layers in the “sandwich” structure,or superlattice structure, through the separating nonmagnetic layerswith the resulting scattering occurring at the layer interfaces, and inthe ferromagnetic layers, being dependent on the electron spins. Themagnetization dependant component of the resistance in connection withthis effect varies as the sine of the absolute value of half the anglebetween the magnetization vectors in the ferromagnetic thin filmsprovided on either side of an intermediate nonmagnetic layer. Theelectrical resistance in the giant magnetoresistance effect through the“sandwich” or superlattice structure is lower if the magnetizations inthe separated ferromagnetic thin-films are parallel and oriented in thesame direction than it is if these magnetizations are antiparallel, i.e.oriented in opposing or partially opposing directions. Further, theanisotropic magnetoresistive effect in very thin films is considerablyreduced from the bulk values therefor in thicker films due to surfacescattering, whereas a significant giant magnetoresistive effect isobtained only in very thin films. Nevertheless, the anisotropicmagnetoresistive effect remains present in the films used in giantmagnetoresistive effect structures.

As indicated above, the giant magnetoresistive effect can be increasedby adding further alternate intermediate nonmagnetic and ferromagneticthin-film layers to extend a “sandwich” structure into a stackedstructure, i.e. a superlattice structure. The giant magnetoresistiveeffect is sometimes called the “spin valve effect” in view of theexplanation that a larger fraction of conduction electrons are allowedto move more freely from one ferromagnetic thin-film layer to another ifthe magnetizations in those layers are parallel than if they areantiparallel or partially antiparallel to thereby result in themagnetization states of the layers acting as sort of a “valve.”

Thus, a digital data memory cell based on the use of structuresexhibiting the giant magnetoresistive effect is attractive as comparedto structures based on use of an anisotropic magnetoresistive effectbecause of the larger signals obtainable in information retrievaloperations with respect to such cells. Such larger magnitude signals areeasier to detect without error in the presence of noise thereby leadingto less critical requirements on the retrieval operation circuitry.

A memory cell structure suitable for permitting the storing andretaining of a digital bit of information, and for permitting retrievingsame therefrom has been demonstrated based on a multiple layer“sandwich” construction in a rectangular solid. This cell has a pair offerromagnetic layers of equal thickness and area separated by aconductive nonmagnetic layer of the same shape and area parallel to theferromagnetic layers but of smaller thickness. These ferromagneticlayers are each a composite layer formed of two strata each of adifferent magnetic material, there being a relatively thin ferromagneticstratum in each of the composite layers adjacent the nonmagnetic layerand a thicker ferromagnetic stratum in each of the composite layersadjacent the thin ferromagnetic stratum therein. The ferromagneticmaterial of the thick stratum in one of the composite layers is the sameas that in the thin stratum in the other composite layer, and theferromagnetic material of the thin stratum in the first composite layeris the same as the ferromagnetic material in the thick stratum of thesecond composite layer. Each of the composite layers is fabricated inthe presence of a magnetic field so as to result in having an easy axisparallel to the long sides of the rectangular solid. The dimensions ofthe cell structure were 10 μm in length and 5 μm in width with anonmagnetic layer of thickness 30 Å. The composite ferromagnetic layersare each formed of a 15 Å thin stratum and a 40 Å thick stratum.

Thus, this memory cell structure has a pair of ferromagnetic layers ofmatching geometries but different magnetic materials in the stratatherein to result in one such layer having effectively a greatersaturation magnetization and a greater anisotropy field than the otherto result in different coercivities in each. In addition, the structureresults in a coupling of the magnetization between the two ferromagneticlayers therein due to exchange coupling between them leading to themagnetizations in each paralleling one another in the absence of anyapplied magnetic fields. As a result, the electrical resistance of thecell along its length versus applied magnetic fields in either directionparallel thereto is represented by two characteristics depending on themagnetization history of the cell. Each of these characteristicsexhibits a peak in this resistance for applied longitudinal fieldshaving absolute values that are somewhat greater than zero, one of thesecharacteristics exhibiting its peak for positive applied longitudinalfields and the other characteristic exhibiting its peak for negativeapplied longitudinal fields. The characteristic followed by theresistance of the cell for relatively small applied longitudinal fieldsdepends on which direction the magnetization is oriented along the easyaxis for the one of the two ferromagnetic layers having the largercoercivity. Thus, by setting the magnetization of the layer with thehigher coercivity, a bit of digital information can be stored andretained, and the value of that bit can be retrieved without affectingthis retention through a determination of which characteristic theresistance follows for a relatively small applied longitudinal field.

Such memory cell behavior for this structure can be modeled by assumingthat the ferromagnetic layers therein are each a single magnetic domainso that positioning of the magnetization vectors in the ferromagneticlayers is based on coherent rotation, and that uniaxial anisotropiescharacterize those layers. The angles of the magnetization vectors inthe two ferromagnetic layers with respect to the easy axis in thoselayers are then found by minimizing the magnetic energy of theseanisotropies summed with that due to the applied external fields and toexchange coupling. That total energy per unit volume is then$\begin{matrix}{E_{Tot} = \quad {E_{1} + E_{2} + E_{12}}} \\{= \quad {{K_{u1}\sin^{2}\theta_{1}} - {M_{s1}H\quad {\cos \left( {\Psi - \theta_{1}} \right)}} +}} \\{\quad {{K_{u2}\sin^{2}\theta_{2}} - {M_{s2}H\quad {\cos \left( {\Psi - \theta_{2}} \right)}} + {A_{12}{{\cos \left( {\theta_{1} - \theta_{2}} \right)}.}}}}\end{matrix}$

Here, K_(u1) and K_(u2) are anisotropy constants, A₁₂ is the exchangeconstant, M_(s1) and M_(s2) are the magnetization saturation values, andH is the externally applied field. As indicated above, once themagnetization vectors have taken an angular position with respect to theeasy axis of the corresponding layer at a minimum in the above indicatedenergy, the effective resistance between the ends of the memory cellstructure is determined by the net angle between the magnetizationvectors in each of these layers.

Because of the assumption of single domain behavior in the ferromagneticlayers, the above equation would seemingly be expected to improve itsapproximation of the assistant total magnetic energy as the length andwidth of that memory cell structure decreased toward having submicrondimensions. However, this mode of operation described for providing thetwo magnetoresistive characteristics based on the history of the layermagnetizations, in depending on the differing anisotropy fields in thetwo ferromagnetic layers because of the differing materials usedtherein, becomes less and less reliable as these dimensions decrease.This appears to occur because decreasing the cell dimensions gives riseto larger and larger demagnetizing fields in the two ferromagneticlayers which, at some point, overwhelm the effects of the anisotropyfields so that the above described behavior no longer occurs asdescribed. In addition, the magnetizations of the two ferromagneticlayers rotate together under the influences of externally applied fieldsat angles with respect to the corresponding easy axis at angularmagnitudes much more nearly equal to one another because of theincreasing demagnetization fields in these layers as the dimensionsthereof decrease. As a result, these ferromagnetic layers are less andless able to have the magnetizations thereof switch directions oforientation independently of one another as the dimensions thereofdecrease so that the structure they are in becomes less able to providethe above described memory function in relying on only theseferromagnetic layer anisotropy differences.

An alternative memory cell structure which is more suited to submicrondimensions is a cell of the kind described above exhibiting “giantmagnetoresistive effect” but which has the two composite ferromagneticlayers formed of different thicknesses in the thick strata therein.Thus, the thick strata in one might be on the order of 40 Å while thatof the other might be on the order of 55 Å as an example. In thisstructure, reducing the size to submicron dimensions uses the shapeanisotropy introduced by this thickness difference to provide differentswitching thresholds for each of the ferromagnetic composite layers inresponse to externally applied operating magnetic fields. The shapeanisotropy leads to the effect of the demagnetizing field of one layeraffecting the switching threshold of the other after the former layerhas switched its magnetization direction. As a result, the thickerferromagnetic layer has a magnetization which is fixed in orientationfor externally applied operating magnetic fields that are justsufficient to switch the thinner ferromagnetic composite layer but notgreat enough to switch the magnetization of the thicker ferromagneticcomposite layer. In effect, the demagnetizing fields as the devicebecomes sufficiently small dominate the anisotropy fields that resultfrom the deposition of the ferromagnetic layers in the presence of amagnetic field.

In the absence of externally applied operating magnetic field, the twocomposite ferromagnetic layers have the magnetizations therein pointingin opposite directions, i.e. they are antiparallel to one another, toresult in the structure as a whole having relatively small celldemagnetizing fields and small external stray fields to affect thenearby memory cells. The direction of magnetization in the thickerferromagnetic composite layer is used to store the digital informationwhich can only be changed in direction by externally applied fieldsgreat enough to switch magnetization directions in both compositeferromagnetic layers. That is, storing new information in the cellrequires that the thicker ferromagnetic layer be capable of having themagnetization direction therein switched to be in accord with theincoming digital data.

Retrieving information from such a memory cell is accomplished byswitching the magnetization direction of the thinner ferromagneticcomposite layer only as a basis for determining in which directionrelative to the thinner layer is the magnetization oriented in thethicker layer. Typically, both such storing and retrieving has meantthat there needs to be a pair of external conductors which cancoincidentally supply current to result in a field large enough toswitch the magnetization of the thicker ferromagnetic composite layer,but with that current in either conductor alone being able to generatefields only sufficient to switch the threshold of the thinnerferromagnetic layer. In some situations, only a single externalconductor need be provided for this purpose because the sense currentused in retrieving information from the memory cell can provide thecoincident current needed with the current in the external conductor toswitch the magnetization direction of the thicker ferromagnetic layer.Such a memory cell is described in an earlier filed co-pendingapplication by A. Pohm and B. Everitt entitled “Giant MagnetoresistiveEffect Memory Cell” having Ser. No. 08/923,478 assigned to the sameassignee as the present application and which is hereby incorporatedherein.

Such a cell formed in a “sandwich” structure would typically exhibit anoutput signal which is on the order of a 5% to 6% change in resistancefrom the nominal resistance of the cell. The retrieval of data from sucha cell typically requires the use of “autozeroing” circuitry which,operated prior to the retrieving step, eliminates retrieval circuitimbalances. This need coupled with the relatively large currents used inthis latter kind of memory cell results in slowing the operation of thatcell, and such currents also lead to substantial power dissipation.Thus, there is a desire for an alternative arrangement for such a“sandwich” structure having submicron dimensions which provide desirablemagnetoresistance versus applied magnetic field characteristics that canbe used for storing and retrieving bits of digital data information butwhich provides a larger signal with less power dissipation so that suchretrievals can be done at a greater rate without undue heat generation.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a digital data memory having a bitstructure in a memory cell based on a dielectric intermediate separatingmaterial with two major surfaces on each of which there is a anisotropicferromagnetic thin film but of differing moments including momentdifferences due to differing thicknesses. The ferromagnetic film at eachsuch surface is a composite film having a thinner stratum of highermagnetic saturation induction adjacent the intermediate material and athicker stratum of lower magnetic saturation induction. These bitstructures are fabricated within structural extent limits to operatesatisfactorily, and can be fabricated to be interconnected in variousinformation retrieval output circuitry configurations. A correspondingconductive word line structure for each of such memory cells ispositioned adjacent the ferromagnetic film on one of these surfaces butseparated therefrom for use in selecting or operating corresponding onesof these memory cells, or both, in data storage and retrievaloperations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B represent a plan view of a portion of a monolithicintegrated circuit structure embodying the present invention and a layerdiagram of a part of this structure,

FIG. 2 represents a fragmentary portion of the layer diagram of FIG. 1B,

FIGS. 3A and 3B represent a plan view of a portion of a monolithicintegrated circuit structure embodying an alternative to the inventionand the layer diagram of a part of this structure shown in FIGS. 1A and1B,

FIG. 4 represents a characteristic diagram for structures similar to oneof those shown in FIGS. 1A, 1B and 2,

FIGS. 5A and 5B represent a plan view of a structure from FIGS 1A, 1Band 2, and an approximation thereof,

FIG. 6 represents a graph of responses for a structure similar to one ofthose shown in FIGS. 1A, 1B and 2,

FIGS. 7A and 7B show graphs of characteristics for a structure similarto one of those shown in FIGS. 1A, 1B and 2,

FIGS. 8A and 8B are a circuit schematic diagram of a portion of adigital memory system based on the structure shown in FIGS. 1A, 1B and2, and an equivalent circuit of a portion of that circuit schematicdiagram,

FIGS. 9A and 9B are a circuit schematic diagram of a portion of analternative digital memory system based on the structure shown in FIGS1A, 1B and 2, and a layer diagram showing additional structure to thatshown in FIGS 1A, 1B and 2,

FIGS. 10A and 10B are a circuit schematic diagram of a portion of analternative digital memory system based on a structure partly similar tothat shown in FIGS 1A, 1B and 2, and a layer diagram showing thatstructure, and

FIGS. 11A, 11B and 11C are circuit schematic diagrams of a portion ofalternative digital memory systems based on the structure shown in FIGS1A, 1B and 2.

DETAILED DESCRIPTION

A digital data bit storage and retrieval memory cell suited forfabrication with submicron dimensions can be fabricated that providesrapid retrievals of bit data stored therein and low power dissipation bysubstituting an electrical insulator for a conductor in the nonmagneticlayer. This memory cell can be fabricated using ferromagnetic thin-filmmaterials of similar or different kinds in each of the magnetic memoryfilms used in a “sandwich” structure on either side of an intermediatenonmagnetic layer which ferromagnetic films may be composite films, butthis intermediate nonmagnetic layer conducts electrical currenttherethrough based primarily on a quantum electrodynamic effect“tunneling” current.

This “tunneling” current has a magnitude dependence on the angle betweenthe magnetization vectors in each of the ferromagnetic layers on eitherside of the intermediate layer due to the transmission barrier providedby this intermediate layer depending on the degree of matching of thespin polarizations of the electrons tunneling therethrough with the spinpolarizations of the conduction electrons in the ferromagnetic layerswhich are set by their magnetization directions to provide a “magneticvalve effect”. Such an effect results in an effective resistance orconductance characterizing this intermediate layer with respect to the“tunneling” current therethrough. In addition, shape anisotropy is usedin such a cell to provide different magnetization switching thresholdsin the two ferromagnetic layers by forming one of the ferromagneticlayers to be thicker than the other. Such devices may be provided on asurface of a monolithic integrated circuit to thereby allow providingconvenient electrical connections between each such memory cell deviceand the operating circuitry therefor.

A “sandwich” structure for such a memory cell, based on having anintermediate thin layer of a nonmagnetic, dielectric separating materialwith two major surfaces on each of which a anisotropic ferromagneticthin-film is positioned, exhibits the “magnetic valve effect” if thematerials for the ferromagnetic thin-films and the intermediate layersare properly selected and have sufficiently small thicknesses. Theresulting “magnetic valve effect” can yield a response which can beseveral times in magnitude greater than that due to the “giantmagnetoresistive effect” in a similar sized cell structure.

FIG. 1A shows a plan view of an example of such memory cells as part ofa digital memory formed as a portion of a monolithic integrated circuit,including a supporting semiconductor chip as part of the memorysubstrate, which can have conveniently provided therein the operatingcircuitry for this memory. FIG. 1B provides a fragmentary view of aportion of the view shown in FIG. 1A to show the layered structurethereof, and also has parts thereof broken out to show some of thestructure there below, again for greater clarity. The protective layerprovided over portions of the structure shown in FIG. 1A in actual usehas been omitted in this view for clarity, but that layer is shown inpart in FIG. 1B. Certain other portions of some layers have beenomitted, again for clarity, so that the structure portions present areshown in solid line form if they are exposed in the absence of somelayer thereover now omitted, but with other structure portions beneaththe solid line form portions appearing in these figures being shown indashed line form.

Corresponding to FIGS. 1A and 1B is FIG. 2 which is a layer diagram of acorresponding portion of the structures shown in FIGS. 1A and 1B. thislayer diagram gives an indication of the structural layers leading toportions of the structures shown in FIGS. 1A and 1B, but FIGS. 1B and 2are not true cross section views in that many dimensions therein areexaggerated or reduced for purposes of clarity.

As indicated above, the memory cell structures in these figures aretypically provided on a semiconductor chip, 10, having suitableoperating circuitry for the memory provided in the resulting monolithicintegrated circuit structure. An electrical insulating layer, 11, formedon semiconductor chip 10 by a sputter deposition of silicon nitride,supports the memory cell “sandwich” structures thereon each of whichcomprises a pair of ferromagnetic thin-film layers that are separatedfrom one another by a nonmagnetic, electrically nonconductive ordielectric intermediate layer, or barrier layer, as will be described inmore detail below. A portion of just layer 11 is shown in the highresolution drawing of FIG. 2. Typically, layer 11 is formed by thissilicon nitride deposited to a thickness of about 10,000 Å. Photoresistis spread over layer 11 and patterned to provide via openingstherethrough and through appropriate ones of the insulating layers inintegrated circuit 10.

A first interconnection, 11′, for the above indicated memory cell“sandwich” structures is next provided on insulating layer 11 as boththis interconnection and as a further substrate portion for supportingthe memory cell “sandwich” structures to be subsequently provided. Thus,a metal deposition is made on layer 11 of aluminum alloyed with 2%copper to cover that layer and fill the via openings therein forelectrical interconnections to the integrated circuitry in and on thesemiconductor substrate below. This metal layer is typically depositedto a thickness of 1000 Å. Photoresist is then spread thereover withopenings therein where the unwanted portions of that metal layer are tobe eliminated, and reactive ion etching is undertaken to provide thiselimination of unwanted metal layer portions. Interconnection andsupport structure 11′ resulting from this elimination is shown in FIGS.1B and 2.

Thereafter, the “sandwich” structures just mentioned are provided oninterconnection support layer 11′ with each of the ferromagneticthin-film layers and the intermediate layer being provided, or at leastinitially provided, through sputter deposition as a basis for forming amagnetoresistive memory cell. This multi layer structure will have avertical direction effective resistivity based on the quantumelectrodynamic effect tunneling current passing therethrough which mightrange from 0.01 to 10,000 MΩ-μm² because of the extreme sensitivity ofthis effective resistivity to the thickness of the barrier layer. Inaddition, the structure will typically exhibit an effective capacitanceand a magnetically controlled tunneling effect response exceeding 20%between the minimum effective resistance value and the maximum effectiveresistance value achievable under such control.

In this structure, the first layer provided is a composite ferromagneticthin-film layer sputter deposited onto interconnection and support 11′with the result shown in FIG. 2. A first stratum, 12, of this compositeferromagnetic thin-film layer is formed of an alloy of 65% nickel, 15%iron and 20% cobalt deposited to a thickness of 40 Å, which has amagnetic saturation induction of typically about 10,000 Gauss, and thisprocess results in the deposited film having a face-centered cubicstructure. The deposition of this layer occurs in the presence of anexternal magnetic field in the plane of the film oriented along adirection parallel to the extended direction of interconnection andsupport 11′ in FIG. 1B This fabrication magnetic field will leave theeasy axis of the film similarly directed. Alternatively, the depositionfield may be provided at an angle to the extended direction ofinterconnection and support 11′ to provide a bias rotation of the layermagnetization to facilitate switching the direction of thatmagnetization.

A second stratum, 13, is also provided in a sputter deposition step inthe presence of a similar fabrication magnetic field. Second stratum 13is formed of 5% iron and 95% cobalt to a thickness of 15 Å resulting inthis material having a magnetic saturation induction of approximately16,000 Gauss which is a higher value than that of the magneticsaturation induction of first stratum 12. This higher saturationmaterial is provided adjacent the intermediate or barrier layer, whichis the next layer to be formed, to thereby obtain a greater magneticallycontrolled tunneling effect, but the lower saturation value in stratum12 is provided to keep the composite film more sensitive to smallerfields than it would be in its absence. This composite layer isdesignated 12, 13 in FIG. 1B.

Thereafter, an intermediate or barrier layer, 14, is provided by sputterdeposition and oxidation onto layer 13, this intermediate layer being adielectric. Layer 14 is begun typically by sputter depositing 12 Å ofaluminum onto layer 13, and continuing to provide two further angstromsof this material using the aluminum sputtering target but alsointroducing oxygen into the sputtering chamber. The result is to convertthe already deposited aluminum layer substantially into aluminum oxidewhich expands its thickness by a factor of about 1.3, and to add anothertwo angstroms of aluminum oxide thereto giving an intermediate layer orbarrier layer thickness of approximately 17.5 Å with the layer beingformed primarily of aluminum oxide. Any portion of the previouslydeposited aluminum metal unoxidized in this process will result in avery thin layer of that aluminum on and between ferromagnetic layer12,13 and the aluminum oxide dielectric barrier layer which can beadvantageous.

The provision of layer 14 is followed by providing a second compositeferromagnetic thin-film layer that is provided on layer 14, and itsstructure matches that of the first composite ferromagnetic layercomprising strata 12 and 13, except for being thinner and reversed instrata order, because of the use of essentially the same depositionsteps. As a result, the stratum having the greater magnetic saturationinduction is again adjacent to layer 14 in the second composite layer,and the lesser magnetic saturation induction stratum is provided thereonbut with a thickness of only 25 Å. Since the strata are otherwise thesame, they have been designated in FIG. 2 as 13′ and 12′ incorrespondence to strata 13 and 12.

After completing this “sandwich” structure, a 2000 Å layer of tantalumor tantalum nitride is sputter deposited on stratum 12′ to passivate toprotect stratum 12′ therebelow, and to allow electrical connectionsthereto for circuit purposes. The resulting layer, 15, is shown inbroken form in FIG. 2 because of its significantly greater thicknesscompared to the ferromagnetic composite layers and the nonmagneticintermediate or barrier layer.

Similarly, a further layer, 16, is deposited on layer 15, and is shownin broken from in FIG. 2 because of its relatively greater thickness of100 Å. Layer 15 is first sputter cleaned which removes around 75 Åthereof. Then, layer 16 is sputter deposited on cleaned layer 15 as achrome silicon layer with 40% chrome and 60% silicon to serve as an etchstop for the subsequent etching of a layer to be provided thereover as amilling mask.

That is, another layer of silicon nitride is next sputter deposited onlayer 16 to a depth of 1000 Å to be used as a milling mask, but thislayer is not shown in FIG. 2 because its remnants will be incorporatedin a further insulating layer to be provided later. On this siliconnitride mask layer, photoresist is deposited and patterned in such a wayas to form a pattern for an etching mask which is to be formed followingthat pattern by leaving the mask portions of the silicon nitride layertherebelow after etching. This last masking pattern in the siliconnitride is to result, after milling therethrough to remove the exposedferromagnetic and nonmagnetic layers therebelow, in a substantial numberof separated bit structures to serve as the memory cells in the digitalmemory each with a “sandwich” construction. Reactive ion etching is usedwith the patterned photoresist to remove the exposed portions of thesilicon nitride masking layer down to chrome silicon layer 16 serving asan etch stop. The remaining portions of the silicon nitride layerprotected by the photoresist serve as the above mentioned milling maskfor the subsequent ion milling step which removes the exposed portionsof chrome silicon layer 16, and thereafter, also the then exposedportions of layer 15, the next exposed portions of the second compositeferromagnetic thin-film layer formed as strata 13′ and 12′, thesubsequently exposed portions of intermediate nonmagnetic layer 14′ and,finally, the resultingly exposed portions of the first compositeferromagnetic thin-film layer formed as strata 13 and 12 down tointerconnection and support 11′.

A portion of one of the resulting memory cells, 17, from FIG. 1A isshown in FIG. 2, as indicated above, and has counterparts thereof shownin FIG. 1B (where they are designated again by numeral 17) with onlysome of the layers in each such cell being represented as distinct inthis latter figure. The full multilayer structure that is shown in FIG.2 with the distinct strata in the composite ferromagnetic layers is notshown in that manner in FIG. 1B because of the larger scale used in thatfigure. Some of these memory cells can also be seen in the plan view ofFIG. 1A, and each of such structures is also designated by numeral 17 inthat figure. The easy axes of the ferromagnetic thin-film compositelayers in each of memory cells 17 are parallel to the direction of thelongest extent of those structures. Each memory cell 17 is formed with arectangular central portion in this plan view continuing into triangularportions tapering away from opposite ends of the rectangular portionalong the easy axis to form the ends of the cell.

Following the completion of memory cell or bit structure 17, anotherlayer of silicon nitride is sputter deposited over those structures andthe exposed portions of interconnection and support 11′ to a thicknessof 7500 Å to form an insulating layer, 19. Photoresist is provided overinsulating layer 19 as an etching mask to provide via openingstherethrough, and through silicon nitride layer 11 and appropriates onesof insulating layers in integrated circuit 10.

On insulating layer 19, so prepared, a further metal deposition is made,again of aluminum alloyed with 2% copper, to cover that layer and fillthe via openings therein, and in silicon nitride layer 11 and theinsulating layers in integrated circuit 10. This metal layer istypically deposited to a thickness of 1000 Å. Photoresist is spreadthereover with openings therein where the unwanted portions of thatmetal layer are to be eliminated, and reactive ion etching is undertakento provide this elimination of unwanted metal layer portions. Thestructures that result from this elimination is shown in FIG. 1B, and inFIG. 1A, as a plurality of upper interconnections, 20, forinterconnecting memory cell structure 17 in parallel to one another inconjunction with interconnection and support 11′. As a result of the viaopenings in silicon nitride layer 11, upper interconnections 20 are alsointerconnected with electronic circuitry in the integrated circuits insemiconductor substrate 10 therebelow.

The completion of upper interconnection structures 20 is followed bydepositing another layer of typically 7500 Å of silicon nitridethereover, and over the exposed portions of silicon nitride layer 20 toform a further insulating layer, 21. Photoresist is provided overinsulating layer 21 as an etching mask to provide via openingstherethrough, and through silicon nitride insulating layers 19 and 11 aswell as though appropriate ones of the insulating layers in integratedcircuit 10.

On insulating layer 21, prepared in this manner, a further metaldeposition is made, again of aluminum alloyed with 2% copper, to coverthat layer and fill the openings therein, and in silicon nitride layers19 and 11 as well as the insulating layers in integrated circuit 10.This metal layer is typically deposited to a thickness of 3500 Å.Photoresist is spread thereover with openings therein where the unwantedportions of that metal layer are to be eliminated, and reactive ionetching is undertaken to provide this elimination of unwanted metallayer portions. The structures that result from this elimination areshown in FIG. 1B, and in FIG. 1A, as a plurality of word lines, 22, forthe memory each positioned across insulating layer 21, upperinterconnection structures 20, and insulating layer 19 fromcorresponding memory cell structures 17 supported on and interconnectedto interconnections and supports 11′. As a result of the via openings,these word lines are also interconnected with electronic circuitry inthe integrated circuits in semiconductor substrate 10 therebelow. Afurther insulator layer, 23, is provided by sputter depositing 7500 Å ofsilicon nitride over word lines 22 and the exposed portions of insulator21. Insulator 23 serves as a passivation and protection layer for thedevice structure therebelow. Layer 23 is seen in FIG. 1B but is notshown in FIG. 1A to avoid obscuring that figure.

A memory cell or bit structure 17 of the structure described resultingfrom the just described process for fabricating same will have arelatively linear change in the quantum electrodynamic effect“tunneling” current therethrough from one ferromagnetic layer to theother with respect to the voltage provided across the cell, i.e. betweenthese ferromagnetic layers, for relatively lower voltages but thecurrent magnitude increases more than linearly for higher values ofvoltage across the cell. As the voltage across the cell increases, thefractional change in the in the “tunneling” current through the cell,for the ferromagnetic layers having magnetizations changing fromparallel to one another to antiparallel, decreases to being only half asgreat with several hundred millivolts across the cell as occurs in thesituation with a hundred or less millivolts across the cell so that thisfractional change with cell voltage will range from a few percent to 20%or more. The fractional change in the resistance of the cell for theferromagnetic layers having magnetizations changing from parallel to oneanother to antiparallel increases to about one and one-half the roomtemperature values when the cell is cooled to 77° K., but the“tunneling” current through the cell increases by only about 10% to 20%indicating that the effective resistivity of the cell is relativelyinsensitive to temperature (around 500 to 1000 ppm/° C.).

The effective resistivity of a cell 17 is set by the amount of“tunneling” current through the cell permitted by barrier layer 14 forthe voltage across the cell. The high sensitivity of the “tunneling”current to the thickness of the barrier layer leads to a wide range ofcell resistivities which have been observed to be from 0.01 to 1000MΩ-μm². On the other hand, barrier layer 14 appears to permit relativelylittle magnetic coupling between the ferromagnetic layers thereacrosswith the coupling fields typically being only a few Oe.

One structural arrangement alternative to that shown in FIG. 1 that ispossible is shown in FIGS. 3A and 3B. In this alternative, the wordlines, now designated 22′, are deposited directly on silicon nitridelayer 11 with an insulating layer, now designated 23′, depositedthereover to provide the supporting substrate for interconnection andsupport 11′ and memory cells 17. Memory cells 17 are again connectedparallel using interconnection and support 11′ on the lower side thereofand upper interconnection 20 on the upper side thereof to provide theinterconnections with memory cells 17 if connected in paralleltherebetween. Insulating layer 21 now becomes the protective andpassivating layer for the device. A further alternative, not shown,would be to provide word lines above and below memory cells or bitstructures 17 at some angle with respect to each other, typically atright angles, for providing magnetic fields to affect the correspondingcell provided between each crossover of such word lines so that currentfor this purpose need not be carried in interconnection and support 11′.

The fabrication steps just described are, of course, applied tosemiconductor material wafers having many integrated circuit chipstherein to serve as memory substrates so that many such digital memoriescan be fabricated simultaneously in and on such wafers. Once all memorycell structures 17 are fabricated on each chip substrate, along with allof the associated interconnection structures and word line structures asprotected by the final insulating layer, the wafers are then ready forwafer testing, for the separating of the individual devices intoseparate chips, and the housing of them in “packages”.

The plan view of the shapes of bit structures 17, i.e. having arectangular center portion tapering into triangular end portions atopposite ends thereof, are not the only plan view geometrical shapeswhich can be used. An alternative would be to form memory cell structure17 with a plan view geometry following a parallelogram. There may beother alternative plan view shapes for memory cell structures 17 whichcan improve the packing density of those structures on an integratedcircuit chip substrate.

A representation of a pair of typical magnetoresistance characteristicsof a memory cell or bit structure 17 versus external magnetic fieldsapplied along its length, i.e. along its easy axis, is shown in FIG. 4for an individual bit structure example of relatively larger size ratherthan a smaller structure taken from a parallel string thereof as a moreeasily understood example. A fixed quantum effect tunneling a current of2.0 μA is used as the operating current through the device between theupper interconnection and the lower interconnection to that device. Thisknown current, along with the measured voltage across the cell, providesthe resistance of that cell.

The characteristic, 30, having the peak on the left in FIG. 4 developsfrom initially having a sufficiently large magnetic field parallel tothe easy axis (shown as a positive field on the plot) applied viacurrent in the adjacent word line 22 and via interconnection and support11′ to force the magnetizations of each of the ferromagnetic thin-films12, 13 and 13′, 12′ in the memory cell or bit structure 17 to beoriented in the direction of the field. These magnetizations will thusbe parallel to one another pointing in an initial common direction tothereby leave the electrical resistance of the cell at a minimum (here,approximately 31 kΩ).

This initial condition is followed by continually reducing this fieldtoward zero and then reversing the field direction, after which themagnitude of the field is continually increased (shown as a negativefield on the plot). As can be seen in the plot, this action begins toincrease the resistance of the cell as the magnetization of thinnerlayer 13′, 12′ begins to rotate toward the opposite direction to agreater degree than does the magnetization of thicker layer 12, 13. Thisdifference occurs because of the shape anisotropy which, as structure 17becomes sufficiently small, dominates the material anisotropy induced bydeposition of the ferromagnetic layers thereof in a magnetic field or bylayer material choice or both.

As a result, these magnetizations begin to be directed more and moreaway from one another as the field gets increasingly negative, therebyincreasing the cell resistance, until the magnetization of the thinnerlayer is rotated just past 90° from the easy axis, whereupon it abruptlyswitches (at approximately −10 Oe) to being significantly directed inthe opposite direction from that of the thicker layer as it attempts toalign with the fields provided by the word line and interconnection linecurrents. At that point, the resistance value correspondingly increasesabruptly to the peak value shown of approximately 37 kΩ. The switchingfield threshold value is set by the bit width and the net magneticmoment of the ferromagnetic layer which in turn is set by magnitude ofthe saturation magnetization and the volume of that layer. Since thevolume, and so the moment, can be chosen by selecting a suitable anddifferent layer thickness with respect to that of the other layer toprovide shape anisotropy, these ferromagnetic layers, even if otherwiseidentical, can have different switching threshold values.

As memory cells 17 are fabricated sufficiently small to be considered ashaving the composite layer ferromagnetic thin-films used therein to besingle magnetic domain structures, the critical magnetic field magnitude(−10 Oe), or threshold, for the thinner layer at which such switchingoccurs (a threshold found much like the well known Stoner Wohlfarththreshold which is defined for larger area films not subject to exchangecoupling and edge effects) is determined from layer magnetic energyconsiderations including the magnitude of the magnetic fieldsestablished by the interconnection structure current in addition to thatestablished by the word line current. (The field due to the operatingcurrent across the intermediate layer can be neglected because thiscurrent is so relatively small). Further magnitude increases in thenegative field do not, however, cause the magnetization of the thickerlayer to switch to being directed in the opposite direction at theexpected Stoner Wohlfarth threshold therefor because the previousswitching of the magnetization direction of the thinner layer inhibitsthe switching of the magnetization of the thicker layer. The change inthe direction of magnetic field occurring in the thicker layer due tothe magnetization of the thinner layer coupled thereto, because ofhaving been previously switched in direction, acts against the switchingof the magnetization of the thicker layer to effectively increase itsswitching threshold.

The magnitude of the magnetic field in the thicker layer due to themagnetization of the thinner layer (and vice versa) depends on thedemagnetization fields in these layers, thus allowing, by selecting theinterconnection structure current magnitude and the memory cell geometryto achieve an appropriate demagnetization factor value, the setting ofthe degree of switching inhibition. That is, the width of the peak incharacteristic 30 can effectively be set by the cell design in operatingconditions. Once this elevated magnitude threshold value (approximately−70 Oe) for the thicker layer is exceeded by the magnitude of theapplied field to force its magnetization direction past 90° from theeasy axis, the magnetization of this layer also switches to result inthe magnetizations of the two ferromagnetic thin-film layers again beoriented in a common direction (although opposite to the initialdirection) to thereby sharply lower the resistance value from the peakvalue of approximately 37 kΩ to the relatively lower value of againabout 31 kΩ. Further increases in the magnitude of the negative field donot significantly further change the resistance value as themagnetization directions in each layer are forced slightly closer andcloser to a common direction. Since the direction of the magnetizationof thicker layer 12, 13 can only be switched by fields having magnitudesgreater than those that switch the magnetization direction of thinnerlayer 13′, 12′, the direction of magnetization of thicker layer 12, 13effectively determines the binary value, “0” or “1”, of the data bitsstored in the cell.

Hence, traversing this large portion of characteristic 30 shown in FIG.4 by changing the externally applied magnetic field due to word linecurrent from a relatively large positive magnitude to a relatively largenegative magnitude in the presence of a sufficient interconnectionstructure current is equivalent to changing the magnetic state of bothlayers from pointing in one direction to pointing in the oppositedirection, i.e. to storing a new data bit by changing the previouslystored data therein based on the direction of the magnetization from itsinitial direction and binary value to another direction and value. Ifthe initially stored data bit value was the same as the new value to bestored, the corresponding increase in the externally applied fieldmagnitude in the opposite direction to store this new data bit, i.e. theincreasing of the field in a positive direction rather than in thenegative direction as described above, would not cause traversing thepeak in characteristic 30 thus leaving the layer magnetization'sdirection and the data bit value unchanged.

The remaining characteristic, 31, in FIG. 4 develops just as didcharacteristic 30 if started from where the development ofcharacteristic 30 terminated as described above, that is, by applying apositively increasing magnitude field in the presence of a sufficientinterconnection structure current based field magnitude after theoccurrence of a large magnitude negative field. Again, the peak in thecell resistance arises in this characteristic by first encountering athreshold like and near to a Stoner Wohlfarth threshold for switchingthinner ferromagnetic layer 13′, 12′ (approximately 8 Oe) to increasethe cell resistance from again about 31 kΩ to around 37 kΩ, andthereafter encountering the elevated magnitude threshold for switchingthicker layer 12, 13 (approximately 58 Oe) to decrease the cellresistance back to about 31 kΩ. Thus, storing a data bit of either a “0”or “1” binary value in a memory cell 17 having the characteristics shownin FIG. 4, as represented by the orientation direction of themagnetization of thicker layer 12, 13 along its easy axis in the schemejust described, requires the application of a sufficient magnitude wordline field in the corresponding direction along that axis in thepresence of a sufficient magnitude interconnection and support structurefield.

Retrieving the stored data without disrupting the value of that data iseasily done in a memory cell or bit structure 17 having characteristics30 and 31, the current one of these characteristics that the cellresistance will follow upon application of interconnection supportcurrent and word line current based fields having been determined by thedirection of orientation of the last external field applied to the cellsufficiently large to switch the magnetizations of both cellferromagnetic thin-film layers in the presence of the choseninterconnection and support line current. A limited externally appliedfield, the limit imposed by limiting the corresponding word line currentand possibly the interconnection and support line current in thepresence of the interconnection and support line current chosen isinitially provided having a value capable of placing the resistance ofthe cell at one of its peak values in either one or the other ofcharacteristics 30 and 31, and the cell structure voltage measuringcircuitry maybe concurrently “autozeroed” to thereby measure a zerovalue in these circumstances.

Such “autozeroing” circuitry and processing need not be used for asingle cell because of the relatively large signal change providedthereby, but the use of several such cells in parallel with one anotherin the circuit to which the voltage measuring circuitry is applied willreduce the output signal of a cell because of the parallel conductionpaths to a value sufficiently small so as to require “autozeroing” ifthe individual cells in that circuit are not electrically isolated fromone another such as by use of a switching arrangement or otherelectrical isolating means. Of course, the ability to dispense with theneed to perform an “autozeroing” step and to eliminate the circuitrytherefor can significantly increase the information retrieval rate inthe memory cell circuit as well as allowing an increase in the densitythereof in a monolithic integrated circuit chip. This initial field islimited in magnitude so as to be unable to switch the direction ofmagnetization of thicker layer 12, 13 in the presence of choseninterconnection and support line current magnitude.

To complete retrieving the stored cell information, the limitedexternally applied field is then reversed from its initial direction toa final limited value in the opposite direction that is capable ofplacing the resistance of the cell at its other characteristicresistance peak, but not capable of switching the direction ofmagnetization of thicker layer 12, 13 in the presence of the choseninterconnection and support line current magnitude. If the cellresistance is actually at a resistance peak initially in following oneof characteristics 30 or 31 because of the direction of the lastpreviously applied word line current base field of a magnitudesufficient to switch magnetizations of both ferromagnetic layers, theresistance after the field reversal will decrease as the magnetizationof the thinner layer switches to be oriented in the same direction asthe direction of magnetization of the thicker layer. If the cellresistance is instead following the other characteristics so that itexhibits a relatively low resistance initially, the resistance after thefield reversal will increase as the magnetization of the thinner layerswitched to be oriented in the direction opposite to the direction ofthe magnetization of the thicker layer.

Thus, the resistance change on the reversal from a field oriented in onedirection of a magnitude limited to be at a resistance characteristicpeak to a limited field oriented in the opposite direction will indicatewhich of the characteristics 30 and 31 the cell followed, and so in whatdirection the last sufficiently large externally applied magnetic fieldwas oriented to thereby indicate the binary value of the data bitrepresented thereby. The change in the resistance value, ΔR, is equal tothe full change in resistance between the peak resistance value,representing the ferromagnetic layers magnetizations being opposed indirection to one another, to the relatively low resistance valuerepresenting the layer magnetizations being oriented in the samedirection as one another. This retrieval process provides a bipolaroutput indication since an increase of this magnitude indicates onestored data bit value and a decrease indicates the opposite stored databit value. Thus, the difference between the magnetic state indicationrepresenting by an increase in resistance, +ΔR, and the magnetic stateindication represented by a decrease in resistance, −ΔR, is+ΔR−(−ΔR)=2ΔR, or twice the resistance change value to give the voltagemeasuring circuitry across the cell structure a readily detectable statedifferentiating output signal to measure from its “auto-zeroed” initialmeasuring point.

The rapidity at which such a binary data retrieval operation can beperformed in such a memory cell or bit structure 17 is initially limitedby the rise time of the currents in interconnection and support 11′ andin word line 22, and by the time required to rotate the magnetizationvectors in the ferromagnetic layers in response to such a current.However, such a minimum data retrieval time limit on the durationrequired to retrieve the data can be further lengthened by the responsetime of the memory cell or bit structure 17 due to the capacitive natureof that cell in having a pair of ferromagnetic conductors on either sideof a dielectric layer yielding an effective capacitance. An importantparameter for memory cell orbit structure 17 is the intrinsicresistance-capacitance time constant of the device due to that barrierlayer 14 resulting therein from the fabrication process of the cell. Thecell capacitance, C, can be approximately determined for the cell from

C=8.85·10⁻¹⁸·8·10⁻⁶A/s

where A is the area in square microns and s is the thickness of thealuminum oxide portion of barrier layer 14 in microns and a dielectricconstant of 8 has been taken as the value appropriate for the aluminumoxide of barrier layer 14. As indicated above, for relatively lowvoltages across the cell (100 mV or less), the effective resistancethrough the cell, R, will typically be on the order of 10⁴ to 10⁹Ω. Theresistance of this portion of the barrier layer can be approximated by

R=k₁se^(k) ^(₂) ^(s)

where k₁ and k₂ are constants characterizing the barrier layer materialand s again is the thickness of that layer in microns. As a result, theresistance-capacitance time constant which is the product of R and Cwill then be exponentially dependent on the thickness s of the aluminumoxide portion of barrier layer 14. This time constant product can bereduced by reducing the thickness of the aluminum oxide portion ofbarrier layer 14 until k₂s is much smaller than 1, or, as a practicalmatter, until difficulties in the fabrication process of thin barrierlayers prevent further reductions in the thickness thereof.

A further lengthening in the minimum time to retrieve data from a memorycell or bit structure 17 will be due to the voltage measuring circuitryacross the cell involving a sensing amplifier to detect the change inthe output voltage of that cell when the magnetizations of theferromagnetic layer are changed from parallel to one another to beingantiparallel. Such a sensing amplifier will have an input resistanceR_(a) typically equal to the combined resistances of the cells connectedthereto to provide approximately the maximum energy transfer from thecell to the amplifier.

If a single cell 17 is operated by a current source providing a currentvalue of 1 therethrough, the voltage across the cell will increase fromI/G_(max) to a value of I/(G_(max)-ΔG) where G_(max)=1/R_(min) if aninformation retrieval operation results in the cell resistance goingfrom R_(min) to R_(min)+ΔR. The effective resistance-capacitance timeconstant of the cell alone in an information retrieval output circuithaving a matching input resistance to provide maximum energy transferfor this increase in voltage forming the cell output signal voltage willbe equal to about C/2G where C includes the capacitance of the cell aswell as the input capacitance of the sensing amplifier. Alternatively,one may instead choose the amplifier input impedance to minimize thenoise generated rather than to maximize the power transfer and, as aresult, increase the time constant by as much as two.

If the aluminum oxide portion of barrier layer 14 has a thickness of 2nm to give a value for C of approximately 0.035 pF (ignoring the inputcapacitance of the sensing amplifier) from the above expressiontherefor, and the cell exhibits a resistance value from 10⁴ to 10⁹Ω fora cell having an area A of one square micron from the above expressiontherefor, the value of the resistance-capacitance time constant for thevoltage rise on the cell through the sensed amplifier would be between0.35 ns and 35,000 ns. The need to be competitive with informationretrieval times of other kinds of random access memory requires that theeffective resistance of a memory cell 17 be much closer to the value of10⁴Ω then to 10⁹Ω to provide competitive data retrieval times, and sothe aluminum oxide portion of barrier layer 14 must be sufficientlythin.

A further parameter to consider with a memory cell or bit structure 17is the associated electrical noise and its relationship to the availablesignal voltage change in switching the magnetizations of theferromagnetic layers therein from anti-parallel to parallel. This signalvoltage is, as can be seen from the foregoing, for a cell 17 with afixed current therethrough that results in approximately 100 mV beingdropped across the cell (the value of voltage above which the responseto a switching of the ferromagnetic layer magnetizations decreases) willbe the cell voltage response to the magnetization direction switchingmultiplied by 100 mV. If the ferromagnetic layer paralleling switchingresponse is 20%, the voltage change signal from the cell will be 20 mV.The noise voltage, on the other hand, is strongly related to theequivalent resistance R of the cell and the bandwidth, Δf, effective inthe information retrieval output circuitry which noise voltage is knownfrom electrical noise theory to at room temperature, be

V_(n)=1.26·10⁻¹⁰F·(R_(min)·Δf)^(½),

where F is the noise factor of the voltage retrieval circuitry systemhaving a minimum value of unity. If the bandwidth Δf is taken to be 100μHz, as is typical in random access memory data retrieval circuitrysystems, the foregoing expression can be written as

V_(n)=1.26·10⁻⁶·F·R_(min) ^(½).

Clearly, the signal-to-noise ratio can be improved by making a memorycell or bit structure 17 of larger area to give a lower resistance, andtherefore a lower noise voltage. For instance, a signal-to-noise ratioof 20, which is a value typically found sufficient for a random accessmemory to provide a low error rate, the above signal value of 20 mVrequires a noise voltage value for V_(n) which is less than 1 mV for anoise factor value of the minimum value 1. This of 1000 μΩ-μm², a memorycell 17 would have to have an area of 1122 square microns. On the otherhand, a cell resistivity of 0.1 μΩ-μm² would result in a memory cellneeding to have an area of only 0.1 square microns. Again, the aluminumoxide portion of barrier layer 14 must be thin enough to permit use of amemory cell having a sufficiently small area to result in a memory on amonolithic integrated circuit chip having a sufficiently high memorycell density.

As to the cell resistance versus applied external magnetic fieldcharacteristics of a cell 17, magnetic structures are known, fromthermodynamic considerations, to have stable equilibrium states ofmagnetization at minimums of the free energies of those structures. Thetotal free energy of a bit structure or memory cell 17 can be givengenerally as

E_(Tot)=E₁+E₂+E₁₂.

where the total free energy is represented by E_(Tot), the self-energyof the first and second layers is represented by E₁ and E₂,respectively, and the interaction energy between the two layers isrepresented by E₁₂. As indicated above, a reasonable approximation forthe structure of a bit structure 17 is to assume that the ferromagneticthin-film layers therein are each of a single domain allowing theassumption that the magnetization of a layer changes only by rotation,and that these layers exhibit uniaxial anisotropy. These and furtherother reasonable approximations, such as assuming there is no magneticenergy at present due to magnetostriction or to other causes and thatthe operating current through barrier or intermediate layer 14essentially perpendicular to the ferromagnetic films provides anegligible magnetic field, allows an analytic representation of thebehavior of a bit structure or memory cell 17 that rather closelymatches the characteristics shown in FIG. 4. The expressions providedfor this purpose representing these energies will be based on the bitstructure shown in FIG. 5A as taken from bit structures 17 shown inFIGS. 1A and 1B.

The self-energy for this purpose of thicker ferromagnetic thin-filmcomposite layer 12, 13, designated here as the first layer, can berepresented as $\begin{matrix}{E_{1} = \quad {{\frac{H_{k1}M_{s1}V_{1}}{2}\quad {\sin^{2}\left( {\theta_{1} - \theta_{S}} \right)}} + {\frac{D_{1L}M_{s1}^{2}V_{1}}{2}\quad {\sin^{2}\left( \theta_{1} \right)}} +}} \\{\quad {{\frac{D_{1w}M_{s1}^{2}V_{1}}{2}\quad {\cos^{2}\left( \theta_{1} \right)}} + {H_{w}M_{s1}^{2}V_{1}{\cos \left( \theta_{1} \right)}} +}} \\{\quad {{H_{b}M_{s1}V_{1}{\sin \left( \theta_{1} \right)}},}}\end{matrix}$

and the self-energy of thinner ferromagnetic thin-film composite layer13′, 12′ considered the second layer can be similarly written as$\begin{matrix}{E_{2} = \quad {{\frac{H_{k2}M_{s2}V_{2}}{2}\quad {\sin^{2}\left( {\theta_{2} - \theta_{s}} \right)}} + {\frac{D_{2L}M_{s2}^{2}V_{2}}{2}\quad {\sin^{2}\left( \theta_{2} \right)}} +}} \\{\quad {{\frac{D_{2w}M_{s2}^{2}V_{2}}{2}\quad {\cos^{2}\left( \theta_{2} \right)}} + {H_{w}M_{s2}V_{2}{\cos \left( \theta_{2} \right)}} -}} \\{\quad {H_{b}M_{s2}V_{2}{{\sin \left( \theta_{2} \right)}.}}}\end{matrix}$

The first term in each of these self-energy expressions represents theanisotropy energy in the corresponding one of the layers due to theanisotropies present therein, primarily the anisotropy brought about bythe deposition of these ferromagnetic films in the presence of amagnetic field leading to requiring energy to deviate the magnetizationof the film from the easy axis therein established by such deposition.The effects of such anisotropies are cumulatively represented in a wellknown manner by an effective anisotropy field in each layer, H_(k1) andH_(k2), respectively, multiplied by the saturation magnetization of thatlayer which is M_(s1) for layer 1 and is M_(s2) for layer 2. The anglebetween the magnetization of the first layer, M₁, shown by a dashed linevector in FIG. 5A, and the average easy axis is represented by θ₁. Theangle between the magnetization of the second layer, M₂, shown by asolid line vector in that figure, and the average easy axis isrepresented by θ₂. The volume of each layer, V₁ for layer 1 and V₂ forlayer 2, multiply the respective terms to give the total anisotropyenergy in each corresponding layer. To provide an initial rotation biasfor the magnetizations in the opposite directions in the twoferromagnetic layers, the easy axis of each layer is rotated on angleθ_(s) during fabrication. Such a bias reduces switching thresholds incells 17.

The second and third terms in each of the last two energy expressionsrepresent the demagnetization self-energy for each of the correspondinglayers in a form typically written therefor. In these terms, the symbolsD_(1L) and D_(2L) each represent the demagnetization factorcorresponding to the length axis for the pertinent one of the first andsecond ferromagnetic thin-film layers in bit structure 17. The symbolsD_(1W) and D_(2W) in these terms each represent the demagnetizationfactors corresponding to the width axis for the related layer.

The fourth terms of each of these expressions represent the energy ofthe magnetization in the corresponding layer due to the current appliedin the associated word line 22 to generate a magnetic field, H_(w),shown directed from right to left in FIG. 5A. Should a meander word linebe used, the field contribution from current in that line would bemerely added to, and part of, the field H_(w) supplied by the currentthrough word line 22 over the corresponding bit structure 17.

The last term in each of these expressions represents the energy of themagnetization in the corresponding layer due to an effective biascurrent being also carried in upper interconnection 20 (which could beinstead carried in interconnection and support 11′) as described aboveif any, this field being designated by H_(b) and represented in FIG. 5Aby an upward pointing solid line arrow assuming current flow in thatbias line is from left to right. Such a bias field can used to aid inswitching the direction of magnetization of the thicker ferromagneticlayer.

The interaction energy between the two ferromagnetic thin-film layers ina bit structure or memory cell 17 is given by $\begin{matrix}{E_{12} = \quad {{{- H_{e}}V_{avg}M_{s - {avg}}\quad {\cos \left( {\theta_{1} + \theta_{2}} \right)}} -}} \\{\quad {{\frac{{V_{1}D_{2w}} + {V_{2}D_{1w}}}{2}\quad M_{s1}M_{s2}{\sin \left( \theta_{1} \right)}{\sin \left( \theta_{2} \right)}} +}} \\{\quad {\frac{{V_{1}D_{sL}} + {V_{2}D_{1L}}}{2}\quad M_{s1}M_{s2}{\cos \left( \theta_{1} \right)}{{\cos \left( \theta_{2} \right)}.}}}\end{matrix}$

The first term in this interaction energy expression accounts for theexchange coupling energy and the correlated surface waviness (texturalvariation) coupling energy which is represented by an effective exchangefield, H_(e), in the usual manner to cover these couplings between theferromagnetic thin-film layers. This term is multiplied by the averagevolume and magnetization of both of these layers, or$V_{avg} = {{\frac{V_{1} + V_{2}}{2}\quad {and}\quad M_{s - {avg}}} = {\frac{{M_{s1}T_{1}} + {M_{s2}T_{2}}}{T_{1} + T_{2}}.}}$

The last two terms in the ferromagnetic layer interaction energyexpression represent the effects of the magnetization in one layer uponthe magnetization in the other analogous to the well known dipole-dipoleinteraction through considering the magnetization in each layer as adipole interacting with the magnetization in the other. Because of theextremely close proximity of the two ferromagnetic layers, the effectivefield in one layer due to the demagnetization field in the other istaken to be identical to that source demagnetization field, ignoring anyeffects of the separation. These terms are responsible for the switchinginhibition described above (and some rotation aiding) leading to theelevated magnitude threshold faced in switching the direction ofmagnetization of thicker ferromagnetic layer 12,13 when themagnetization of thinner layer 13′,12′ is directed oppositely to that ofthe thicker layer.

Ferromagnetic material masses of general shapes in a magnetic field leadto very complex demagnetization factors characterizing the internalresponse of that mass to the field. Homogeneous bodies having surfacescharacterizable by expressions in the second degree lead todemagnetization factors in uniform fields which are much more tractable,i.e. the magnetization factors for ellipsoids have been determined inanalytical closed form. As can be seen in FIG. 5B, an ellipsoid can beprovided with corresponding ones of its axes numerically equal to thelength, width and thickness of the ferromagnetic thin-film layers in bitstructure 17 of FIG. 5A. The resulting ellipsoid can be seen to ratherclosely approximate those layers in viewing FIGS. 5A and 5B together, atleast for thinner composite ferromagnetic layer 13′, 12′ exposed in FIG.5A. However, the length and width of thicker ferromagnetic compositethin-film layer 12,13 is the same as that of layer 13′, 12′ and thethickness differences between the two layers are easily accounted for bythe different dimensions of the third axis of the ellipsoids used torepresent them to reflect the difference between the thicknesses ofthose layers. The resulting demagnetization factors are$D_{1L} = {{\frac{4\quad \pi \quad {\cos (\varphi)}{\cos \left( \zeta_{1} \right)}}{{\sin^{3}\left( \zeta_{1} \right)}{\sin^{2}\left( \alpha_{1} \right)}}\quad\left\lbrack {{F\left( {k_{1},\zeta_{1}} \right)} - {E\left( {k_{1},\zeta_{1}} \right)}} \right\rbrack}\quad {and}}$$D_{2L} = {\frac{4\quad \pi \quad {\cos (\varphi)}{\cos \left( \zeta_{2} \right)}}{{\sin^{3}\left( \zeta_{2} \right)}{\sin^{2}\left( \alpha_{2} \right)}}\quad\left\lbrack {{F\left( {k_{2},\zeta_{2}} \right)} - {E\left( {k_{2},\zeta_{2}} \right)}} \right\rbrack}$

for the demagnetization factors corresponding to the lengths of theellipsoids representing the two ferromagnetic thin-film layers of memorycell 17 in FIG. 5A, and $\begin{matrix}\begin{matrix}{D_{1W} = \quad {\frac{4\quad \pi \quad {\cos (\varphi)}{\cos \left( \zeta_{1} \right)}}{{\sin^{3}\left( \zeta_{1} \right)}{\sin^{2}\left( \alpha_{1} \right)}{\cos^{2}\left( \alpha_{1} \right)}} \times}} \\{\quad \left\lbrack {{E\left( {k_{1},\zeta_{1}} \right)} - {{\cos^{2}\left( \alpha_{1} \right)}{F\left( {k_{1},\zeta_{1}} \right)}} - \frac{{\sin^{2}\left( \alpha_{1} \right)}{\sin \left( \zeta_{1} \right)}{\cos \left( \zeta_{1} \right)}}{\cos (\varphi)}} \right\rbrack}\end{matrix} \\{and} \\\begin{matrix}{D_{2W} = \quad {\frac{4\quad \pi \quad {\cos (\varphi)}{\cos \left( \zeta_{2} \right)}}{{\sin^{3}\left( \zeta_{2} \right)}{\sin^{2}\left( \alpha_{2} \right)}{\cos^{2}\left( \alpha_{2} \right)}} \times}} \\{\quad \left\lbrack {{E\left( {k_{2},\zeta_{2}} \right)} - {{\cos^{2}\left( \alpha_{2} \right)}{F\left( {k_{2},\zeta_{2}} \right)}} - \frac{{\sin^{2}\left( \alpha_{2} \right)}{\sin \left( \zeta_{2} \right)}{\cos \left( \zeta_{2} \right)}}{\cos (\varphi)}} \right\rbrack}\end{matrix}\end{matrix}$

for the demagnetization factors corresponding to the widths of theseellipsoids for these layers.

In these equations for the demagnetization factors, the varioustrigonometric terms, cos(φ), cos(ζ_(x)), and sin(α_(x)), are defined as$\begin{matrix}{{{\cos (\phi)} = {\frac{W}{L}\quad \left( {0 \leq \phi \leq \frac{\pi}{2}} \right)}},} \\{{{\cos \left( \zeta_{x} \right)} = {\frac{T_{x}}{L}\quad \left( {0 \leq \zeta_{x} \leq \frac{\pi}{2}} \right)}},\quad {and}} \\{{\sin \left( \alpha_{x} \right)} = {\left\lbrack \frac{1 - \left( \frac{W}{L} \right)^{2}}{1 - \left( \frac{T_{x}}{L} \right)^{2}} \right\rbrack^{\frac{1}{2}} = {\frac{\sin (\varphi)}{\sin \left( \zeta_{x} \right)} = {k_{x}\quad {\left( {0 \leq \alpha_{x} \leq \frac{\pi}{2}} \right).}}}}}\end{matrix}$

where L is the length of the ellipsoid, W is the width of the ellipsoid,and T_(x) represents the thicknesses of either of the two approximatingellipsoids through x being set to either 1 or 2 to indicate the first orsecond layer. The functions denoted by F(k_(x), ζ_(x)) and E(k_(x),ζ_(x)) are elliptic integrals of the first kind and the second kind,respectively, defined as $\begin{matrix}{{F\left( {k_{x},\zeta_{x}} \right)} = {\int_{0}^{\zeta_{x}}{\frac{1}{\sqrt{1 - {k_{x}^{2}{\sin^{2}(\psi)}}}}\quad {\psi}\quad {and}}}} \\{{E\left( {k_{x},\zeta_{x}} \right)} = {\int_{0}^{\zeta_{x}}{\sqrt{1 - {k_{x}^{2}{\sin^{2}(\psi)}}}\quad {\psi}}}}\end{matrix}$

These expressions for the demagnetization factors can be furthersimplified in several situations, including the present situation wherethe length and width are of comparable magnitudes but each are more thanan order of magnitude greater than the thickness, i.e.

L≧W>>T_(x).

In this circumstance, the demagnetization factors can be expressed as$\begin{matrix}{D_{1L} = {{4\quad \pi \quad T_{1}\quad \frac{\left( {1 - e^{2}} \right)^{\frac{1}{2}}}{L}\quad \frac{K - E_{c}}{e^{2}}} = {4\quad \pi \quad T_{1}F_{L}\quad {and}}}} \\{D_{2L} = {{4\quad \pi \quad T_{2}\quad \frac{\left( {1 - e^{2}} \right)^{\frac{1}{2}}}{L}\quad \frac{K - E_{c}}{e^{2}}} = {4\quad \pi \quad T_{2}F_{L}}}}\end{matrix}$

for the demagnetization factors corresponding to the lengths of theellipsoids representing the corresponding one of the two ferromagneticlayers, and $\begin{matrix}{D_{1W} = {{4\quad \pi \quad T_{1}\quad \frac{E_{c} - {\left( {1 - e^{2}} \right)K}}{{{Le}^{2}\left( {1 - e^{2}} \right)}^{\frac{1}{2}}}} = {4\quad \pi \quad T_{1}F_{W}\quad {and}}}} \\{D_{2W} = {{4\quad \pi \quad T_{2}\quad \frac{E_{c} - {\left( {1 - e^{2}} \right)K}}{{{Le}^{2}\left( {1 - e^{2}} \right)}^{\frac{1}{2}}}} = {4\quad \pi \quad T_{2}F_{W}}}}\end{matrix}$

for the demagnetization factors corresponding to the widths of theseellipsoids representing those layers. In these last equations for thedemagnetization factors, the symbol e is defined as$E = {\left( {1 - \frac{W^{2}}{L^{2}}} \right)^{\frac{1}{2}}.}$

The functions K and E_(c) are complete elliptic integrals given as$\begin{matrix}{K = {\int_{0}^{\frac{\pi}{2}}{\frac{1}{\sqrt{1 - {e^{2}{\sin^{2}(\psi)}}}}\quad {\psi}\quad {and}}}} \\{E_{c} = {\int_{0}^{\frac{\pi}{2}}{\sqrt{1 - {e^{2}\sin^{2}}}\quad {{\psi}.}}}}\end{matrix}$

Using these last expressions for the demagnetization factors, the aboveexpressions for the energy components in the free energy of the bitstructures 17 of FIG. 5A can be rewritten to in simplified form. Theself-energy of ferromagnetic thin-film layer 12,13 becomes$\begin{matrix}{E_{1} = \quad {{\frac{H_{k1}M_{s1}V_{1}}{2}\quad {\sin^{2}\left( {\theta_{1}\theta_{s}} \right)}} + {\frac{4\quad \pi \quad T_{1}F_{L}M_{s1}^{2}V_{1}}{2}\quad {\sin^{2}\left( \theta_{1} \right)}} +}} \\{\quad {{\frac{4\quad \pi \quad T_{1}F_{W}M_{s1}^{2}V_{1}}{2}\quad {\cos^{2}\left( \theta_{1} \right)}} + {H_{w}M_{s1}V_{1}{\cos \left( \theta_{1} \right)}} +}} \\{\quad {H_{b}M_{s1}V_{1}{{\sin \left( \theta_{1} \right)}.}}}\end{matrix}$

The expression for the self-energy of layer 13′,12′ now is$\begin{matrix}{E_{2} = \quad {{\frac{H_{k2}M_{s2}V_{2}}{2}\quad {\sin^{2}\left( {\theta_{2} - \theta_{s}} \right)}} + {\frac{4\quad \pi \quad T_{2}F_{L}M_{s2}^{2}V_{2}}{2}\quad {\sin^{2}\left( \theta_{2} \right)}} +}} \\{\quad {{\frac{4\quad \pi \quad T_{2}F_{W}M_{s2}^{2}V_{2}}{2}\quad {\cos^{2}\left( \theta_{2} \right)}} + {H_{w}M_{s2}V_{2}{\cos \left( \theta_{2} \right)}} -}} \\{\quad {H_{b}M_{s2}V_{2}{{\sin \left( \theta_{2} \right)}.}}}\end{matrix}$

Finally, the interaction energy expression becomes $\begin{matrix}\begin{matrix}{E_{12} = \quad {{{- H_{e}}V_{avg}M_{avg}\quad {\cos \left( {\theta_{1} + \theta_{2}} \right)}} -}} \\{\quad {{\frac{{V_{1}4\pi \quad T_{2}F_{w}} + {V_{2}4\quad \pi \quad T_{1}F_{w_{1}}}}{2}\quad M_{s1}M_{s2}{\sin \left( \theta_{1} \right)}{\sin \left( \theta_{2} \right)}} +}} \\{\quad {\frac{{V_{1}4\pi \quad T_{2}F_{L}} + {V_{2}4\quad \pi \quad T_{1}F_{L}}}{2}\quad M_{s1}M_{s2}{\cos \left( \theta_{1} \right)}{\cos \left( \theta_{2} \right)}}}\end{matrix} \\{or} \\\begin{matrix}{E_{12} = \quad {{{- H_{e}}V_{avg}\quad M_{avg}{\cos \left( {\theta_{1} + \theta_{2}} \right)}} -}} \\{\quad {{\frac{{V_{1}T_{2}} + {V_{2}T_{1}}}{2}\quad 4\pi \quad F_{w}M_{s1}M_{s2}{\sin \left( \theta_{1} \right)}{\sin \left( \theta_{2} \right)}} +}} \\{\quad {\frac{{V_{1}T_{2}} + {V_{2_{1}}T}}{2}\quad 4\pi \quad F_{L}M_{s1}M_{s2}{\cos \left( \theta_{1} \right)}{{\cos \left( \theta_{2} \right)}.}}}\end{matrix}\end{matrix}$

Some limitations must be satisfied by bit structures 17 to have thembehave during digital data storage and retrieval operations as desired.The width dimension W thereof is subject to at least two conditionswhich limit its extent to being less than certain values. As indicatedabove in describing the operating process, the switching of thedirections of the magnetizations of the layers at certain thresholdvalues of the word line current based fields depends on the magnitude ofthe bias fields due to the bias current since these bias fields affectthose thresholds. The ability of the fields generated by the biascurrents to affect the thresholds will be diminished and eventuallydisappear or become negligible as the width dimension of bit structure17 become increasingly wide. This begins to occur when the width of thebit structure exceeds twice the curling length experienced in thatstructure for the magnitude of the word line current based field usedfor switching the directions of the layer magnetizations.

The magnetizations of portions near the sides of a bit structure 17 arepinned there in a direction parallel to the sides to a greater extentthan those in the interior of the structure because of demagnetizationconsiderations. Thus, as the structure widens, the applied field will atsome point switch the magnetization of the layers in the interiorcentral portions thereof without having switched the magnetizations ofthe layer portions near the edges thereof. As a result, the fields dueto the bias current, which are orthogonal in direction to those inducedby the word line currents, tend to interact with the magnetizations ofthe ferromagnetic layers in two counteracting ways rather than in onesupporting way. That is, the fields due to the bias current will causetorques on the magnetizations already rotated in the central interiorportions in one direction, but cause torques on the magnetizations atthe edges in an opposite direction to thereby in effect cancel out thetorques on the magnetizations. The intent is instead to have the torquesgenerated by the bias current fields act in a common direction to aidthe switching of the magnetizations under the applied word line currentfields. Hence, the width of bit structure 17 should be no greater thantwice such curling lengths occurring therein from its sides inward. Ifthe width of a bit structure 17 is less than twice the curling length'scharacteristic thereof, the magnetizations of the layers, both in thecentral interior portions and near the outer side portions thereof,rotate together and so are subject to torques from the field generatedby the bias current in a common direction.

On the other hand, the bias current can be kept sufficiently small so asnot to be a significant factor in determining the magnetizationdirection switching thresholds, particularly when a meander word line isused. In any event, suitable operation of bit structure 17 as describedabove requires that demagnetizing fields be of sufficient magnitude toserve to inhibit the switching of the thicker ferromagnetic thin-filmlayer magnetization once the magnetization of the thinner layer hasalready been oppositely directed as described above. This inhibition isdue primarily to the demagnetizing field components that parallel thewidth dimension of bit structure 17 since these demagnetizing fieldswill be considerably greater in magnitude than those along the length ofthat structure given that the width has significantly smaller extentthan does the length. To assure that the demagnetizing fields associatedwith the width are dominant with respect to the anisotropy fieldsoccurring in the layers, a bit structure 17 is required to have a widthsufficiently small to result in the width component demagnetizationfield being larger than the anisotropy field for the layer, or thatH_(dx), representing the demagnetization field along the widths, begreater than H_(kx) leading to

H_(dx)=D_(xw)M_(sx)=4πT_(x)F_(w)M_(sx)>H_(kx).

This inequality then yields${F_{w} > \frac{H_{kx}}{4\pi \quad T_{x}M_{sx}}},\quad {{{or}\quad \frac{E_{c} - {\left( {1 - e^{2}} \right)K}}{{{Le}^{2}\left( {1 - e^{2}} \right)}^{\frac{1}{2}}}} > {\frac{H_{kx}}{4\pi \quad T_{x}M_{sx}}.}}$

Since K and E_(c) do not vary much in value with decreasing values of e,the ratio of the width to the length must be kept sufficiently smallsuch that the e² factor in the denominator on the left side of theinequality is small enough to make the inequality hold for the choice ofthickness of the corresponding ferromagnetic layer material chosen, thismaterial having a particular anisotropy field H_(kx) and a particularsaturation magnetization M_(sx).

Aside from its effect on the ratio of width to length in the lastequality, there are no significant further limitations on the length ofa bit structure 17 beyond practicality limitations. One such limitationis that the need to provide a fairly uniform word line current basedmagnetic field across a bit structure 17 requires that the word line beapproximately as wide as that bit structure. For a desired magnitude ofthe field generated by the word line current, that current necessary toprovide that field magnitude becomes proportional to the width of theword line, that is, equivalently, to the length of the bit structure 17.Thus, there is not only a desire to keep the bit length short to allowas many as possible to be provided in the digital memory to improve itsdensity of storage, there is also a desire to keep the word linecurrents as small as possible to reduce the heat dissipation in themonolithic integrated circuit which also, as just indicated, requireskeeping the bit structure lengths relatively short.

A final limitation on bit structure 17 is related to the requireddifference in thicknesses between thicker composite ferromagneticthin-film layer 12,13 and thinner layer 13′,12′. An insufficientdifference in thickness between these two layers in the face of highrate of applied magnetic field changes, such as results from the use ofan abrupt change in current in the word lines to generate such fieldchanges, can lead to switching the direction of magnetization in bothlayers concurrently even though just one of them was intended to beswitched. That is, if the difference in thickness between the layers,and so the difference in magnetization therebetween, is not sufficientlylarge, the direction of magnetization in each of these layers willswitch together even if the magnitude intended for the switching fieldis only slightly larger than the switching threshold value of thethinner layer should the change in that field be applied at a highenough rate. Such high rates of application will result from the typicalchanges in word line current values resulting from transistor switchesbeing switched off and on which often result in the current changeapproximating a step function.

Since the behavior of the magnetization in the ferromagnetic layers of abit structure in response to a sharply changing magnetic field is ofpresent concern, the equations of motion for the magnetizations in theferromagnetic layers in response to changes in magnetic fields arepertinent. Suitable equations of motion are found to relate the timerate of change of the magnetization to the torque applied to thatmagnetization by all of the magnetic fields present plus some damping ofthat motion. A well known equation expressing this relationship, basedon choosing a certain form of such damping, is the Gilbert equation or${\frac{}{t}\quad \overset{\rightarrow}{M}} = {{\gamma \left( {\overset{\rightarrow}{M} \times \overset{\rightarrow}{H}} \right)} - {\frac{\alpha}{M_{s}}{\left( {\overset{\rightarrow}{M} \times \frac{}{t}\quad \overset{\rightarrow}{M}} \right).}}}$

Here, the magnetization is shown as a vector, {right arrow over (M)}, asis the total magnetic field applied to the magnetization, {right arrowover (H)}. The symbol γ is the gyromagnetic ratio, and the symbol α isthe damping constant which will be quite small, typically in the rangeof 0.02 to 0.04.

Expressing this vector equation in its components represented inspherical coordinates results in the following coupled system of firstorder differential equations (for ferromagnetic thin-films withmagnetizations constrained by vertical demagnetizing fields toessentially lie in the plane of the corresponding film) yields$\begin{matrix}{{{\frac{}{t}\quad \theta} = {{\frac{4\quad \pi \quad M_{s}{\gamma }}{1 + \alpha^{2}}\quad \varphi} + {\frac{{\gamma }\alpha}{M_{s}\left( {1 + \alpha^{2}} \right)}\quad \tau}}},\quad {and}} \\{{\frac{}{t}\quad \varphi} = {{\frac{4\quad \pi \quad M_{s}\alpha {\gamma }}{1 + \alpha^{2}}\quad \varphi} - {\frac{\gamma }{M_{s}\left( {1 + \alpha^{2}} \right)}\quad {\tau.}}}}\end{matrix}$

Here, θ is the azimuthal angle (or ferromagnetic layer in-plane angle)and φ is the polar angle (or ferromagnetic layer out of plane angle) inspherical coordinates. The symbol τ represents the torque effectivelyapplied as a result of the magnetic fields present.

The response of the magnetizations of the thinner and thickerferromagnetic thin-films in a bit structure 17 to the dynamicapplication of torques via the magnetic fields generated by theassociated word line currents can be found from these latter equations.FIG. 6 shows a graph of the rotational angular responses of themagnetizations to a word line current generated torque shifting abruptlyfrom zero to a value of about 30 Oe. The lower coercivity thinner layer13′,12′ is represented by an upper plot, 40, on that graph showing howit is more responsive to the applied torque than is the highercoercivity, thicker layer 12,13 represented by the lower plot designated41. This upper plot is shown with the thinner layer magnetization havingrotated in response to an angle from the easy axis of about π radians,and so this plot reflects that the direction of the magnetization isswitched in that layer due to the application of the word line currentgenerated field.

Notice that there is a substantial oscillation in the angular positionof this magnetization vector in reaching its final angular value as seenin plot 40. Although this oscillation, or “ringing”, in the response ofthe magnetization of the thinner ferromagnetic layer to the stepfunction in the word line field is relatively inconsequential to theoperation of the device, a similar “ringing” occurs in the rotationalangular behavior of the magnetization vector of the thicker layer asseen in plot 41 which can be of much more significance. The largest peakin the “ringing” portion of the lower plot reaches an angular value thatis more than twice the angular value of the final angular position takenby the thicker layer magnetization as a result of the applied stepfunction word line current change and the aiding effect of themagnetization angular position change in the thinner layer (up to thepoint of switching the magnetization direction in that layer).

Although not a problem for the bit structure represented in FIG. 6because the switching of the magnetization direction in the thinnerlayer immediately thereafter begins to inhibit the angular positionalchange of the magnetization of the thicker layer before it reaches π/2radians, in some bit structures having too small a difference betweenthe thicknesses of the ferromagnetic thin films or too small a sensecurrent, or both, this initial peak in the “ringing” could reach the π/2radian value. At that point, the thicker ferromagnetic layer switchesthe direction of its magnetization more or less concurrently with theswitch in the direction of the magnetization of the thinner layer eventhough the ultimate final value intended for the angle of rotation ofthe magnetization of the thicker layer in response to the step functionword line current change was less than π/2 radians. That is, a word linecurrent step function could be applied with the intention of switchingthe direction of magnetization of the thinner ferromagnetic layer butnot that of the thicker layer, but nevertheless result in switching thedirections of magnetizations of both layers because of the peak in the“ringing” portion of the response of the magnetization of the thickerlayer to the applied field.

The implications of this dynamic behavior of the rotating magnetizationsof the thinner or thicker ferromagnetic thin-films in bit structure 17can be seen in FIGS. 7A and 7B. In these figures, the dynamic thresholdsare plotted as opposed to the quasi-static thresholds which result fromminimizing the foregoing energy equations without regard to the dynamicbehavior of the structure during operation. The quasi-static thresholdswould be from 10% to 15% greater than the dynamic switching thresholdsshown in FIGS. 7A and 7B. These switching thresholds are all found onthe basis of using the highest rate of change in the effectively appliedtorque, that is, they are the thresholds which result from astep-function change in the word line current.

The lowest threshold, 50, represents the switching threshold for thinnerlayer 13′,12′ when both the magnetization thereof, and of the thickerlayer, have not yet been switched to a direction that is the same as thedirection of the applied word line field. The next greater switchingthreshold, 51, is also for the thinner ferromagnetic thin-film layer,but with the thicker layer having its direction of magnetizationmatching that of the applied word line field. As can be seen, themagnetization of the thicker layer in this instance has inhibited therotation of the magnetization of the thinner layer to effectively raisethe switching threshold for that thinner layer. In each instance, thethinner layer switching threshold is plotted as a function of the layerthickness difference but that layer has the same thickness of 40 Å forall the plots on this graph.

The next greater switching threshold shown, 52, is the dynamic thresholdfor thicker layer 12,13 when both this thicker layer and the thinnerlayer have the magnetizations thereof opposed to the direction of theapplied word line field. As a result, there is no inhibiting effect fromthe magnetization of the thin layer on the magnetization change of thethicker layer so that both switch as a result of the applied field.However, switching threshold 52 is shown as a function of increasingthickness of the thicker layer to thereby result in an increasingthickness difference between the thicker and the thinner layer. Clearly,the switching threshold increases significantly for the thicker layer asthe difference of thicknesses in the two layers increases. The finalswitching characteristic, 53, represents the situation in which thethinner layer is already switched to be directed in the same directionas the applied word line field so as to inhibit the switching of themagnetization of the thicker layer. Again, switching characteristic 53is plotted as a function of the increasing thickness difference betweenthe layers.

As can be seen, the dynamic switching threshold plots for the thinnerlayer cross the dynamic switching threshold plots for the thicker layeras the thickness differences between the two layers sufficientlydecrease. In this situation, there is clearly a substantial risk ofhaving the directions of magnetization in both the thicker and thinnerlayer switch together in response to the application of a step functionchange in the word line current. As the thickness difference between thelayers increases, an increasing gap develops between the switchingthresholds for the thinner layer and the switching thresholds for thethicker layer thus providing a margin of safety in avoiding theswitching of the thicker layer in response to a step-function change inthe word line current intended to switch only the direction of themagnetization of the thinner layer.

FIG. 7B shows dynamic switching thresholds of the same nature as thoseshown in FIG. 7A except they are found in connection with the use of agreater sense current, so in FIG. 7B the threshold designation numeralsmatching those used in FIG. 7A are followed by a prime mark to result inbeing designated 51′, 52′, 53′ and 54′. Switching thresholds in FIG. 7Bare established on the basis of an initial bias angle θ_(S-2) which, ineffect, provides a field of 25 Oe in opposite directions in each layer,as compared to the bias angle θ_(S-1) used in finding the switchingthresholds shown in FIG. 7A where the effective field due thereto wasonly 15 Oe. Clearly, the use of a larger magnitude bias angle results ina greater safety margin at lower thickness differences between thethinner layer switching thresholds and the thicker layer switchingthresholds.

Thus, the thickness difference between the ferromagnetic thin-films usedin a bit structure 17 must be sufficiently great, for the magnitude ofthe bias angle and the structure geometrical parameters used, to assurethat the application of a word line field intended to switch the thinnerlayer does not also have the effect of an unintended switching of thethicker layer also. In many situations, the thickness difference betweenthe ferromagnetic thin-film layers will need to exceed at least 10% ofthe average of these two thicknesses.

The energy equations above, based on the ellipsoidal approximationsdescribed there for bit structure 17 meeting the foregoing limitations,can be minimized to find the equilibrium angular positions of theferromagnetic layer magnetizations as a function of the applied bias andword line current generated fields, and to find the quasi-static fieldthresholds. Necessary conditions for such an energy minimum are$\tau_{2} = {{\frac{\partial}{\partial\theta_{2}}\quad E_{Tot}} = {{0\quad {and}\quad \tau_{1}} = {{\frac{\partial}{\partial\theta_{1}}\quad E_{Tot}} = 0.}}}$

Taking the derivative with respect to θ₁ to find the torque τ₁ andsetting the result equal to zero as indicated in the equation for thattorque above yields $\begin{matrix}{\tau_{1} = \quad {{H_{k1}M_{s1}{\sin \left( {\theta_{1} - \theta_{2}} \right)}{\cos \left( {\theta_{1} - \theta_{2}} \right)}} +}} \\{\quad {{4\pi \quad M_{s1}^{2}{T_{1}\left( {F_{L} - F_{W}} \right)}V_{1}{\sin \left( \theta_{1} \right)}{\cos \left( \theta_{1} \right)}} -}} \\{\quad {{H_{w}M_{s1}V_{1}{\sin \left( \theta_{1} \right)}} + {H_{b}M_{s1}V_{1}{\cos \left( \theta_{1} \right)}} +}} \\{\quad {{H_{c}\quad \frac{T_{avg}M_{avg}V_{1}}{T_{1}}\quad {\sin \left( {\theta_{1} + \theta_{2}} \right)}} -}} \\{\quad {{T_{2}4\pi \quad F_{w}M_{s1}M_{s2}V_{1}{\cos \left( \theta_{1} \right)}{\sin \left( \theta_{2} \right)}} -}} \\{\quad {T_{2}4\pi \quad F_{L}M_{s1}M_{s2}V_{1}{\sin \left( \theta_{1} \right)}{\cos \left( \theta_{2} \right)}}} \\{= \quad 0}\end{matrix}$

using V_(x)=AT_(x), with A being the surface area of the ferromagneticlayers, and V_(avg)=AT_(avg) where the parameter T_(avg) is$T_{avg} = {\frac{T_{1} + T_{2}}{2}.}$

Similarly, the derivative with respect θ₂ of the total energy to providethe torque τ₂ and setting it equal to zero as indicated above yields$\begin{matrix}{\tau_{2} = \quad {{H_{k2}M_{s2}{\sin \left( {\theta_{2} - \theta_{2}} \right)}{\cos \left( {\theta_{2} - \theta_{2}} \right)}} +}} \\{\quad {{4\pi \quad M_{s2}^{2}{T_{2}\left( {F_{L} - F_{W}} \right)}V_{2}{\sin \left( \theta_{2} \right)}{\cos \left( \theta_{2} \right)}} -}} \\{\quad {{H_{w}M_{s2}V_{2}{\sin \left( \theta_{2} \right)}} - {H_{b}M_{s2}V_{2}{\cos \left( \theta_{2} \right)}} +}} \\{\quad {{H_{e}M_{s - {avg}}V_{2}\quad \frac{T_{avg}}{T_{2}}\quad {\sin \left( {\theta_{1} + \theta_{2}} \right)}} -}} \\{\quad {{T_{1}4\quad \pi \quad F_{w}M_{s1}M_{s2}V_{2}{\sin \left( \theta_{1} \right)}{\cos \left( \theta_{2} \right)}} -}} \\{\quad {T_{1}4\quad \pi \quad F_{L}M_{s1}M_{s2}V_{2}{\cos \left( \theta_{1} \right)}{\sin \left( \theta_{2} \right)}}} \\{= \quad 0}\end{matrix}$

A possibility for determining the equilibrium angles and the switchingthresholds is to use one of these torque equations to eliminate thedependence in the total energy equation on either one of themagnetization direction rotational angles, and then find the secondderivative of the energy and set it to zero to determine the point inwhich the system is going from a stable equilibrium to an unstable one,i.e. the switching point, yielding$\frac{^{2}{E_{tot}\left( \theta_{x} \right)}}{\theta_{x}^{2}} = 0.$

Alternatively, the two torque equations can be solved self-consistentlyto obtain the desired solutions. Furthermore, the resistancecharacteristics for the corresponding bit structure can be plottedversus applied word line current generated field which closely matchthose shown in FIGS. 3, 4A and 4B using these results and$R = {R_{o} + {\Delta \quad R\quad {{\sin \left( \frac{\theta_{1} - \theta_{2}}{2} \right)}.}}}$

To determine whether such a resistance change has occurred as the resultof applying external magnetic fields to a memory cell, memory cells orbit structures 17 will be grouped for purposes of applying operatingcurrent thereto for information retrieval operations, and for connectingthem to information retrieval output circuitry, to provide efficient useof surface areas in which such cells and such circuitry is provided inthe monolithic integrated circuit structure. Thus, a succession of Nmemory cell or bit structure 17 could be connected in a series stringthereof, for instance, and supplied operating current of a magnitude Iflowing through that series string to result in a voltage dropthereacross equal to INR_(min) where R_(min) is the minimum resistanceof each cell absent applied external magnetic fields or at the extremevalues of the external magnetic fields being used. The minimumresistance of each cell will be taken to equal that of all the othersfor simplicity.

A change in resistance of one selected cell in the series string becauseof applied external fields of a magnitude sufficient to switch thedirection of magnetization of the thinner ferromagnetic layer in thatcell will be taken to have a value of ΔR in reaching its peak resistancevalue. Such an increase in resistance will result in an output signal ofa magnitude equal to I(NR_(min)+ΔR−NR_(min)) or ΔRI which can be writtenIR_(min)ΔR/R_(min) or IR_(min) where r is the resistive response ratioof an individual cell, i.e. ΔR/R_(min). In these circumstances, thesignal-to-noise ratio of such a series string of memory cells, s/n|_(s),can be written${\frac{s}{n}}_{s} = \frac{{IR}_{\min}r}{{1.26 \cdot 10^{- 6}}{F\left( {NR}_{\min} \right)}^{1/2}}$

using the noise voltage found above for a single cell adapted for N suchcells in a series string thereof. This series string of memory cells orbit structures 17 results in a relatively large voltage being developedthereacross, INR_(min), for a significant operating current Itherethrough because of the high resistance of the barrier orintermediate layer 14 in each such cell, and the electrical noise in theseries string increases with N^(½) so that the signal-to-noise ratiodecreases by the factor N^(−{fraction (1/12)}). Furthermore, the seriesstring will have a relatively high impedance which the informationretrieval circuitry will have to match with its input impedance toprovide a maximum power transfer.

Monolithic integrated circuits are continually being operated at lowerand lower supply voltages, and therefore, may often be more compatiblewith a parallel interconnection of N memory cells or bit structures 17.Furthermore, limiting the voltage drop across each such cell to 100 mVto obtain the maximum fractional voltage response, as indicated above,is more easily done in a parallel connection of such cells than a seriesinterconnection if there is substantial resistance differences from onecell to another. Furthermore, the interconnection of cells in parallelreduces the effective impedance of the interconnected string which insome situations may be more compatible with the input impedance of theoutput information retrieval circuitry.

Such a parallel interconnected arrangement is shown in FIG. 8A Here, twosequences of memory cells or bit structures 17 out of an array of manyare shown with the cells in each sequence being connected in parallelwith one another, an upper sequence which is connected to a first outputinformation retrieval circuit sensing amplifier, 60, and a lowersequence of parallelly interconnected cells is connected to a furtherinformation retrieval output circuit sensing amplifier, 61. The twooutputs of each of sensing amplifiers 60 and 61 provide complementarylogic signals on a pair of data output lines, 62 and 63. Sensingamplifier 60 is enabled by a first enable line, 64, and sensingamplifier 61 is enabled by a second enable line, 65.

Each of memory cells or bit structures 17 in the upper sequence thereofhas a word line 22 passing by it, and each is connected between upperinterconnection 20 and interconnection and support 11′. In switchingcircuitry not seen, interconnection and support 11′ and interconnection20 can be connected to carry an operating current I of which a fractionis provided to each cell 17 connected therebetween during informationretrieval processes while their corresponding sensing amplifier isenabled, and while the word line adjacent the cell selected forretrieval has current directed therethrough sufficient to switch themagnetization direction of thinner ferromagnetic layer 12′, 13′ therein.For information storage processes, interconnection and support 11′ andthe corresponding one of word lines 22 can be switched to both havesubstantial currents passed therethrough that together are sufficient toprovide a magnetic field to switch the direction of magnetization ofthicker layer 12, 13 in the corresponding one of cells 17.

FIG. 8B shows a basic equivalent circuit for one of the interconnectedparallel sequences of memory or bit structures 17 depicted in FIG. 8A,the upper sequence being chosen for illustration in FIG. 8B. Each ofcells 17 is represented by a resistance having a value of R_(min) and acapacitance having a value of C. One of the cells 17, in addition tobeing represented as having a resistance of R_(min), has had itsmagnetic state changed so as to be further represented as exhibiting anadditional resistance of value ΔR above its minimum value of R_(min).Again for simplicity, all cells are represented as having the sameresistance and capacitance values except for the one exhibiting theadded resistance value of ΔR. Sense amplifier 60 is shown having aninternal impedance represented by a resistor with a value of resistanceR_(a) and a capacitance having a capacitance value of C_(a). In thiscircuit arrangement for interconnecting a group of cells 17 to oneanother and the information retrieval output circuitry, the effectiveresistance-capacitance time constant would be with R_(a)=1/G_(a)$\frac{{NC} + C_{a}}{{NG}_{\max} + G_{a}},$

which, for small C_(a) and maximum energy transfer so that Ra=Rmin/N,again becomes$\frac{NC}{{NG}_{\max} + {NG}_{\max}} = {\frac{C}{2G_{\max}}.}$

The resistance of N parallel resistances of value R_(min) is equal toR_(min)/N. The resistance value of (N−1) resistors of value R_(min) inparallel with each other and in parallel with a further resistance ofvalue R_(min)+ΔR is$\frac{R_{\min}^{2} + {R_{\min}\Delta \quad R}}{{NR}_{\min} + {\left( {N - 1} \right)\Delta \quad R}}.$

As a result, the output signal of the sequence of memory cells 17connected in parallel when one of them has an increase in resistance ofΔR is${I\left\lbrack {\frac{R_{\min}^{2} + {R_{\min}\Delta \quad R}}{{NR}_{\min} + {\left( {N - 1} \right)\Delta \quad R}} - \frac{R_{\min}}{N}} \right\rbrack}.$

This can instead be written as${\frac{{IR}_{\min}}{N}\quad \frac{\frac{\Delta \quad R}{R_{\min}}}{N + {\left( {N - 1} \right)\frac{\Delta \quad R}{R_{\min}}}}} = {\frac{{IR}_{\min}}{N}\frac{r}{N + {\left( {N - 1} \right)r}}}$

where r is again the response ratio ΔR/R_(min).

If a voltage V_(o) of approximately 100 mV is to be nominally maintainedacross each of memory cell 17 in the parallelly interconnected sequencethereof to keep them at their maximum output signal sensitivity, asindicated above, the nominal voltage drop IR_(min)/N across thatinterconnected array can be written as V_(o) to give${V_{o}\quad \frac{\frac{\Delta \quad R}{R_{\min}}}{N + {\left( {N - 1} \right)\frac{\Delta \quad R}{R_{\min}}}}} = {V_{o}\quad {\frac{r}{N + {\left( {N - 1} \right)r}}.}}$

Thus, the output signal of the array of parallel interconnected memorycells decreases as the number of memory cells increases, that is, themagnetoresistive response ratio r of an individual memory celleffectively is decreased as the number of memory cells in parallelincreases giving an output signal that decreases from around 20 mV foran individual cell to around 2 mV if 10 such cells are connectedtogether in parallel.

The signal-to-noise ratio of the array of parallel interconnected memorycell s becomes${\frac{s}{n}}_{p} = {\frac{V_{o}\quad \frac{r}{N + {\left( {N - 1} \right)r}}}{{1.26 \cdot 10^{- 6}}{F\left( \frac{R_{\min}}{N} \right)}^{1/2}}.}$

As can be seen, the electrical noise is reduced in the array of parallelinterconnected memory cells by a factor of (I/N)^(½) due to the memorycells being connected in parallel which leads to a reduced effectiveresistance for the sequence as the source of electrical noise therein.This reduced noise, of course, is countered by the reduced value of theoutput signal so that the signal-to-noise ratio also decreases by thefactor (N)^(−½) (actually, a bit more than that depending on the valueof the response ratio).

Such circumstances will, in many instances, require finding some otherkind of circuit arrangement in which to interconnect a group of memorycells or bit structures 17 with information retrieval output circuitrywhich does not lead to reducing the signal at the output from a singlecell 17 of around 20 mV to some effectively much smaller signal value inthe presence of a group of interconnected cells dependent on the numberof cells so interconnected. One way of preserving this output signalvalue of around 20 mV from each memory cell or bit structure 17 in anarray thereof to the information retrieval output circuitry is shown inFIG. 9A. There, n-channel enhancement mode metal-oxide-semiconductorfield-effect transistors (MOSFET's), 70, are provided in and onsemiconductor substrate 10 each in series with a corresponding one ofmemory cells or bit structures 17. In such an arrangement, thesetransistors are able to, when in the “off” condition, substantiallyelectrically isolate the corresponding one of those cells from thecorresponding conductor leading therefrom to the information retrievaloutput circuitry, and so from the other cells in the group or array.

Each of transistors 70 has its source, 71, electrically connectedthrough an interconnection, 72, to a corresponding one of cells 17. Eachof transistors 70 has its drain, 73, electrically connected to aconductor, 74, which in turn is electrically connected to allow theswitching thereof to an electrical energization source, not shown, whichsupplies operating current to any selected one of cells 17 connectedthrough its corresponding transistor 70 thereto. Conductor 74 is alsoconnected to a selection MOSFET, 75, serving as a pass transistor whichin turn is connected to sensing amplifier 60. Each of selectiontransistors 75 is switched into the “on” condition by a correspondingsignal on the corresponding one of the gate selection interconnections,76, connected to its gate.

The opposite side of each of cell 17 is electrically connected throughan interconnection, 77, to a further conductor, 78, which in turn iselectrically connected to allow the switching thereof to an electricalenergization source, not shown, which either draws operating currentfrom any selected one of cells 17 connected thereto, or supplies amagnetic field generating current for aiding in the switching of themagnetization direction of a ferromagnetic layer or layers in a cell 17.In the latter instance, the magnetic field generated by a current inconductor 77 can aid the magnetic field generated by current in a wordline 22 passing near one of cells 17 of interest to have either or bothmagnetic fields available for information retrieval or informationstorage purposes. Transistors 70 are switched between the “on” conditionand the “off” condition by selection signals applied to their gates, 79,over corresponding conductors, 80.

Transistors 70 can alternatively each be interchanged in position withits corresponding cell 17 in the series connection thereof between aconductor 74 and a conductor 78 to place that cell in the transistordrain circuit rather than its source circuit without significant effecton the electrical isolation of the cell from other cells. Also, bipolartransistors could alternatively be used for transistors 70 rather thanthe MOSFET's shown. Of course, the implementing structures in monolithicintegrated circuits for these alternatives would require substantialchanges from that used with the MOSFET devices shown.

Coincident currents are used for selecting a cell 17 as the basis forretrieving information therefrom or for storing information therein. Toretrieve information from a cell 17, a signal is provided over conductor80 to gate 79 of the transistor to which that cell is connected toswitch that transistor into the “on” condition, and an electricalenergization source is switched onto conductor 74 to provide operatingcurrent through the transistor and the corresponding selected cell whichis carried away on conductor 78. A current in the corresponding one ofword lines 22 for that cell is generated sufficient to switch themagnetization of the thinner one of the ferromagnetic layers in the cellas part of the retrieval operation process. The switching on oftransistor 70 allows a resistance shift in cell 17 to cause an increasedvoltage drop thereacross which is sensed through that transistor 70 online 74 by sensing amplifier 60 which receives that signal through thecorresponding one of selection transistor 75 selected by a signal on thecorresponding one of selection line 76.

An information storage operation, on the other hand, requires coincidentcurrents being switched onto conductor 78 and word line 22 whichtogether provide a sufficiently large magnetic field to switch themagnetization direction of the thicker one of the ferromagnetic layersin the selected one of cells 17 at which the currents are coincidentallypresented. Transistors 70 are switched into the “off” condition duringsuch storage operations.

One possible construction for a portion of a monolithic integratedcircuit chip to provide the circuit arrangement shown in FIG. 9A isindicated in the layer diagram for one of cells 17 and the associatedtransistor 70 shown in FIG. 9B. There, semiconductor material substrate10 of p-type conductivity has transistor 70 formed in and on a portionof a major surface thereof which is made available by providing anopening in a device isolating field oxide, 81. Drain 73 andinterconnection 74 are formed by an implanted n⁺-type conductivityregion in semiconductor substrate 10, and separated from a similarn⁺-type conductivity implanted region forming source 71 by dopedpolysilicon gate 79 extending to doped polysilicon conductor 80 not seenin this figure. Gate 79 is separated from semiconductor substrate 10 bya thin gate oxide, 82. Transistor 70 as described is formed by wellknown techniques for the fabrication of such transistors in monolithicintegrated circuits.

A further insulating oxide layer, 83, has been provided over transistor70, and an opening has been provided therein over source 71 in and overwhich a first metal deposition is provided. This result is and subjectedto a planarization process to leave a metal plug of aluminum alloyedwith 2% copper as interconnection 72 extending through oxide layer 83from source 71 to the upper surface of this oxide layer. On this metalplug, or interconnection, 72 is formed a memory cell or bit structure17, in the manner described above. Using well known techniques followingsuch a provision, a layer of silicon nitride. 84, has been deposited andthen patterned to provide a opening therein to memory cell 17 in which asecond metal deposition and patterning has been provided to forminterconnection 77 and conductor 78, again an aluminum alloy with 2%copper. Another layer of silicon nitride, 85, is then deposited as abase for the subsequent deposition of a metal layer to form word line22, again of aluminum alloyed with 2% copper. Finally, a passivating andprotective layer of silicon nitride, 86, is deposited.

In an alternative cross-point interconnection arrangement forinformation retrieval output circuitry interconnecting groups of cells17, coincident currents are again used in both information retrievaloperations undertaken to retrieve information from those cells, and ininformation storage operations used in connection with such cells. Sucha circuit arrangement is shown in FIG. 10A where conductors 74 are nowconnected directly to one side of the succession of memory cells or bitstructures 17, and are connectable to an electrical energization sourceat one end thereof to provide either operating currents or a magneticfield generating current for those cells connected thereto. In addition,conductors 74 are also connected to a corresponding one of passtransistors 75 as in FIG. 9A which can each be switched into the “on”condition by signals provided on the corresponding selection line 76connected to its gate to thereby provide a conductive path from thecorresponding conductor 74 to sensing amplifier 60. Conductors 78 arethis time shown extending in a direction substantially perpendicular toconductor 74, and they can carry away operating current from a selectedcell 17 electrically connected thereto or can carry current to generatea magnetic field thereabout which will affect those cells 17electrically connected thereto.

If cells 17 were each directly connected to a corresponding conductor78, so as to each be directly connected between a corresponding one ofconductors 74 and a corresponding one of conductors 78, the connectingof an electrical energization source to a selected conductor 74 and aselected conductor 78 would lead to current in other current pathsthrough the array beyond the current path through the cell 17 connectedbetween these two selected conductors. Current in such other currentpaths can lead to undesirable effects with respect to the output signalfrom the intended selected cell directly connected between the selectedconductors. Such unwanted effects can again be alleviated by providingsufficient electrical isolation of each cell 17 from the other cellspresent in the array so that substantially the full 20 mV output signalavailable from the selected cell will reach sensing amplifier 60 throughthe corresponding enabled one of transistors 75.

One way this can be accomplished is shown in FIG. 10A by connecting adiode, 90, between each cell 17 and its connection to a correspondingconductor 78 with the diode anode connected through an interconnection,91, to cell 17. This prevents any conductive path portions extendingfrom conductors 78 through corresponding cells 17 to the correspondingconductors 74. Alternatively, the positions of each of diodes 90 and itscorresponding cell can be interchanged in the series connection betweena conductor 74 and a conductor 78 without significant effect on theelectrical isolation of a cell from the other cells in theinterconnected group, although a different structural implementationwould be used in a monolithic integrated circuit. Thus, in thisalternative, the cathode of the diode would be connected to itscorresponding one of cells 17.

FIG. 10B shows a portion of a layer diagram of one of cells 17 and thecorresponding diode of FIG. 10A formed in a well known manner along withthe interconnections 74 and 78 extending substantially perpendicular toone another also formed by known techniques. Semiconductor substrate 10of p-conductivity first has a n⁺-type conductivity region, 92, implantedtherein in an opening in field oxide 81 to prevent any transistor actionoccurring between substrate 10 and the regions to be subsequentlyprovided in that substrate above region 92. This region is designated92, 78 because it will also form part of conductor 78. Thereafter, afurther n-type conductivity region, 93, is implanted through that sameopening in oxide 81 over and into region 92 to complete the diodecathode and also again form part of conductor 78. This is followed byimplanting a p-type conductivity region, 94, through the same oxideopening over and into region 93 to form the diode anode. An oxide layer,95, is then provided over this field oxide opening, oxide layer 81 andregion 94, and an opening is thereafter provided in oxide layer 95 toprovide an access opening to region 94. A first metal layer of aluminumalloyed with 2% copper is then deposited and patterned to fill thisopening to provide a metal plug, or interconnection, 91, with the resultsubjected to a planarization process. A memory cell or bit structure 17is then formed on metal plug 91 in the manner described above.

Thereafter, using known techniques, a silicon nitride layer, 96, isdeposited over the exposed surfaces of metal plug 91, oxide layer 95 andcell 17. An opening is provided in silicon nitride layer 96 to reachcell 17 and metal is deposited in a second metal deposition step, againaluminum alloyed with 2% copper, to provide conductor 74 afterpatterning. A further passivating and protective silicon nitride layer,97, is then provided.

Small memories which are embedded in other circuitry being usedtherewith often require larger output signals than are provided bymemory cells or bit structures 17 directly. Such cells can beincorporated in a flip-flop arrangement to provide the current steeringnecessary to place the flip-flop in a selected state. Two such flip-flopcircuits are shown in FIGS. 11A and 11B.

FIG. 11A shows a pair of n-channel enhancement mode MOSFET's, 100 and101, each having its source electrically connected to a ground referencepotential terminal, 102, provided in conjunction with a positive voltagepower supply terminal, 103, suited for connection to a source ofpositive voltage. Each of transistors 100 and 101 has its drainconnected to the source of a corresponding one of a pair of n-channelenhancement mode MOSFET's, 104 and 105, serving as loads in the draincircuits of the corresponding ones of transistors 100 and 101. Loadtransistors 104 and 105 each has its gate connected to its drain. Thesedrains in turn are connected to positive supply voltage terminal 103.

The side of the circuit of FIG. 11A having transistors 100 and 104connected in series with one another, and the side of that circuithaving transistors 101 and 105 connected in series with one another, arecross-coupled to one another through use of two memory cells or bitstructures 17. Each of cells 17 has one side thereof connected to acorresponding one of the drains of transistors 100 and 101, and has theother side thereof connected to the gate of the opposite one of thosetransistors. In addition, a word line 22 extends past each of cells 17but carries the current therethrough in opposite directions by each ofthose cells so as to be able to switch the magnetization directions inthe ferromagnetic layers of each cell in a direction opposite to that ofthe other. A further word line, 22′, is also provided past of each ofcells 17 so that cells 17 can be selected for magnetization directionchanges of the thicker ferromagnetic layer therein through the use ofcoincident currents in word lines 22 and 22′.

Since of each cells 17 are in the opposite storage state in having themagnetizations of the thicker layers therein pointing in oppositedirections with respect to fields generated by currents in word line22′, a switching of voltage from the ground potential to a substantialpositive level on positive voltage supply terminal 103 coincident withcurrent through word line 22′ will result in one of cells 17 being inthe minimum resistance condition and the other in the maximum resistancecondition. As a result, a greater current will pass through the cellwith the smaller resistance into the parasitic capacitance extending toground from the gate of the corresponding one of transistors 100 and 101to which it is connected. This parasitic gate capacitance will thus gaincharge more rapidly to thereby switch that transistor to the “on”condition, and so prevent the opposite one of those transistors fromswitching into that condition because of the decrease in the drainvoltage of the “on” transistor to a value below the threshold voltage ofthe opposite transistor having its gate connected to this drain as this“on” transistor draws current through its load. The two differentvoltage levels at the drains of transistors 100 and 101 after such aswitching of sufficient positive voltage onto terminal 103 represent thedigital information and its complement stored in the thickerferromagnetic layers of the cells 17.

The flip-flop will maintain this condition for the positive voltage onterminal 103 being maintained even after current is removed from wordline 22′ since the voltage at the drain of that one of transistors 100and 101 which is switched into the “on” condition will continue to be ofa value less than the threshold voltage of the other. This output resultof switching from ground potential to a sufficient positive voltage onterminal 103 along with the coincidental provision of current in wordline 22′ can be changed for the next such switching only by havingpreviously coincidentally provided word line currents through word line22′ and word line 22 to thereby have switched the magnetizations of thethicker one of the ferromagnetic layers in each of cells 17.

A similar operation is achieved in the complementarymetal-oxide-semiconductor field-effect transistor (CMOS) circuit of FIG.11B when the voltage on positive supply voltage terminal 103 isincreased from ground potential to a sufficient positive potential, butthe dependence on the parasitic capacitance of the gates of transistors100 and 101 is not as significant because of the changed configuration.Transistors 104 and 105 in FIG. 11A are replaced with a pair ofp-channel enhancement mode transistors, 104′ and 105′, in FIG. 11B withthe gates thereof electrically connected to the corresponding gates oftransistors 100 and 101, respectively. The gates of transistors 100 and104′ are cross-coupled to the other side of the circuit through beingdirectly connected to the drain of transistor 105′, and the gates oftransistors 101 and 105′ are similarly cross-coupled by being directlyconnected to the drain of transistor 104′. Cells 17 are each connectedbetween the source of one of transistors 104′ and 105′ and the drain ofthe corresponding one of transistors 100 and 101.

Thus, as positive voltage is switched onto positive supply terminal 103with a coincident current flowing in word line 22′, the one of cells 17having the larger resistance will lead to a greater voltage drop acrossit and the corresponding one of transistors 100 and 101 to the drain ofwhich it is connected so as to switch into the “on” condition first theopposite one of those transistors. A temporary current will flow throughthis “on” condition transistor, the cell 17 connected to its drain andthe p-channel transistor connected to that cell with this lattertransistor than switching into the “off” condition. This will leave theother p-channel transistor in the “on” condition and the n-channeltransistor in series therewith and the other cell 17 in the “off”condition. Again, this output result of switching from ground potentialto a sufficient positive voltage on terminal 103 along with thecoincidental provision of current in word line 22′ can be changed forthe next such switching only by having previously coincidentallyprovided word line currents through word line 22′ and word line 22 tothereby have switched the magnetizations of the thicker one of theferromagnetic layers in each of cells 17.

The circuit shown in FIG. 11C has the cells 17 repositioned from thedrain circuits of transistors 100 and 101, as shown in FIG. 11B, to thesource circuits of those transistors to each be connected between acorresponding one of those sources and ground. Much the same operationwill occur for this circuit as the for the circuit in FIG. 11B exceptthat the one of cells 17 having the larger resistance leading to agreater voltage drop across it will prevent the one of transistors 100and 101 connected thereto from switching into the “on” condition firstresulting in the other of those transistors doing so.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

What is claimed is:
 1. A ferromagnetic thin-film based digital memory,said memory comprising: a plurality of bit structures interconnectedwith information retrieval circuitry having a plurality of transistorsso that each said bit structure has a said transistor electricallycoupled thereto that selectively substantially prevents current in atleast one direction along a current path through that bit structure,each said bit structure comprising: an electrically insulativeintermediate layer, said intermediate layer having two major surfaces onopposite sides thereof; and a memory film of an anisotropicferromagnetic material on each of said intermediate layer major surfaceshaving switching thresholds for magnetizations of said film adjacenteach of said intermediate layer major surfaces that differ in value fora switching of these magnetizations from both being directed initiallyat least in part in substantially a common direction to being directedat least in part in substantially opposite directions versus a switchingfrom being directed initially at least in part in substantially oppositedirections to both being directed at least in part in substantially acommon direction; and a plurality of word line structures each having apair of word line end terminal regions adapted to conduct electricalcurrent in at least one direction therethrough, each of said pairs ofword line end terminal regions having an electrical conductorelectrically connected therebetween which is located across anelectrical insulating layer from said memory film on one of said majorsurfaces of said intermediate layer of a corresponding one of said bitstructures.
 2. The apparatus of claim 1 wherein a said bit structure hasa length along a selected direction and a width substantiallyperpendicular thereto that is smaller in extent than said length, saidmemory film in a said bit structure being characterized by an anisotropyfield, and said width being sufficiently small that demagnetizationfields arising in said memory film in response to its saturationmagnetization being oriented along that said width exceed in magnitudesaid anisotropy field.
 3. The apparatus of claim 1 wherein a said bitstructure has a length along a selected direction and a widthsubstantially perpendicular thereto that is smaller in extent than saidlength and has a shaped end portion extending over a portion of saidlength in which said width gradually reduces to zero at an end thereof.4. The apparatus of claim 1 wherein said memory film at each of saidmajor surfaces of said intermediate layer of at least one of said bitstructures is arranged such that there are two separate films with oneof said separate films on each of said major surfaces.
 5. The apparatusof claim 1 wherein said plurality of word line structures each hascorresponding positional relationships with plural ones of saidplurality of bit structures.
 6. The apparatus of claim 1 wherein saidintermediate layer major surfaces adjacent said memory film having asurface area sufficiently large to provide at least that signal-to-noiseratio needed by said information retrieval circuitry to permitdeterminations thereby of directions of magnetizations of said memoryfilm on each of said intermediate layer surfaces.
 7. The apparatus ofclaim 4 wherein said bit structure has a length along selected directionand a width substantially perpendicular thereto that is smaller inextent than said length, said width being less than about two curlinglengths of said separate films from edges thereof substantiallyperpendicular to said width.
 8. The apparatus of claim 4 wherein saidmemory film at each of said major surfaces is a composite film having athinner stratum of higher magnetic saturation induction adjacent theintermediate material and a thicker stratum of lower magnetic saturationinduction.
 9. The apparatus of claim 4 wherein said separate memoryfilms at each of said major surfaces has differing magnetic momentsbecause of thickness differences therein.
 10. A ferromagnetic thin-filmbased digital memory, said memory comprising: a plurality of bitstructures interconnected with information retrieval circuitry so as toselectively provide current in at least one direction along a currentpath through at least some set of said plurality of bit structuresduring an information retrieval from a said bit structure, each said bitstructure comprising: an electrically insulative intermediate layer of aselected thickness, said intermediate layer having two major surfaces onopposite sides thereof, and a memory film of an anisotropicferromagnetic material on each of said intermediate layer major surfaceshaving switching thresholds for magnetizations of said film adjacenteach of said intermediate layer major surfaces that differ in value fora switching of these magnetizations from both being directed initiallyat least in part in substantially a common direction to being directedat least in part in substantially opposite directions versus a switchingfrom being directed initially at least in part in substantially oppositedirections to both being directed at least in part in substantially acommon direction, said intermediate layer major surfaces adjacent saidmemory film having a surface area which, with said intermediate layerthickness, together provide a time constant for electrical energizationsof said bit structures that is less than 35 μs; and a plurality of wordline structures each having a pair of word line end terminal regionsadapted to conduct electrical current in at least one directiontherethrough, each of said pairs of word line end terminal regionshaving an electrical conductor electrically connected therebetween whichis located across an electrical insulating layer from said memory filmon one of said major surfaces of said intermediate layer of acorresponding one of said bit structures.
 11. The apparatus of claim 10wherein a said bit structure has a length along a selected direction anda width substantially perpendicular thereto that is smaller in extentthan said length, said memory film in a said bit structure beingcharacterized by an anisotropy field, and said width being sufficientlysmall that demagnetization fields arising in said memory film inresponse to its saturation magnetization being oriented along that saidwidth exceed in magnitude said anisotropy field.
 12. The apparatus ofclaim 10 wherein a said bit structure has a length along a selecteddirection and a width substantially perpendicular thereto that issmaller in extent than said length and has a shaped end portionextending over a portion of said length in which said width graduallyreduces to zero at an end thereof.
 13. The apparatus of claim 10 whereinsaid memory film at each of said major surfaces of said intermediatelayer of at least one of said bit structures is arranged such that thereare two separate films with one of said separate films on each of saidmajor surfaces.
 14. The apparatus of claim 10 wherein said plurality ofword line structures each has corresponding positional relationshipswith plural ones of said plurality of bit structures.
 15. The apparatusof claim 10 wherein said intermediate layer major surfaces adjacent saidmemory film having a surface area sufficiently large to provide at leastthat signal-to-noise ratio needed by said information retrievalcircuitry to permit determinations thereby of directions ofmagnetizations of said memory film on each of said intermediate layersurfaces.
 16. The apparatus of claim 13 wherein said bit structure has alength along selected direction and a width substantially perpendicularthereto that is smaller in extent than said length, said width beingless than about two curling lengths of said separate films from edgesthereof substantially perpendicular to said width.
 17. The apparatus ofclaim 13 wherein said memory film at each of said major surfaces is acomposite film having a thinner stratum of higher magnetic saturationinduction adjacent the intermediate material and a thicker stratum oflower magnetic saturation induction.
 18. The apparatus of claim 13wherein said separate memory films at each of said major surfaces hasdiffering magnetic moments because of thickness differences therein.